revert(rds): document DatabaseInstance.fromDatabaseInstanceAttributes() (aws#12091)

Reverts aws#11979

This commit broke java compilation of the module:

```console
#STDOUT> [ERROR] COMPILATION ERROR : 
#STDOUT> [INFO] -------------------------------------------------------------
#STDOUT> [ERROR] /tmp/npm-packP61iji/monocdk/src/main/java/software/amazon/awscdk/services/rds/DatabaseInstance.java:[38,5] method does not override or implement a method from a supertype
```

Basically the `fromDatabaseInstanceAttributes` function now exists both in `DatabaseInstanceBase` and `DatabaseInstance`, and for some reason the one in the child is annotated with an `@Override`, despite being a static function. (see more details below).

Since this module is stable, I don't want to simply move the function, but I do want to bring our pipeline back to life quickly, so this seems like an ok compromise for now.

-----------------

What happens is that the generated java code for some reason adds an `@Override` annotation to the function signature of the concrete class, as if its overriding the function in the base class, even though these are static methods.

```java
@javax.annotation.Generated(value = "jsii-pacmak/1.15.0 (build 585166b)", date = "2020-12-15T17:40:10.678Z")
@software.amazon.jsii.Stability(software.amazon.jsii.Stability.Level.Stable)
@software.amazon.jsii.Jsii(module = software.amazon.awscdk.services.rds.$Module.class, fqn = "@aws-cdk/aws-rds.DatabaseInstance")
public class DatabaseInstance extends software.amazon.awscdk.services.rds.DatabaseInstanceBase implements software.amazon.awscdk.services.rds.IDatabaseInstance {

    protected DatabaseInstance(final software.amazon.jsii.JsiiObjectRef objRef) {
        super(objRef);
    }

    protected DatabaseInstance(final software.amazon.jsii.JsiiObject.InitializationMode initializationMode) {
        super(initializationMode);
    }

    /**
     * @param scope This parameter is required.
     * @param id This parameter is required.
     * @param props This parameter is required.
     */
    @software.amazon.jsii.Stability(software.amazon.jsii.Stability.Level.Stable)
    public DatabaseInstance(final @org.jetbrains.annotations.NotNull software.constructs.Construct scope, final @org.jetbrains.annotations.NotNull java.lang.String id, final @org.jetbrains.annotations.NotNull software.amazon.awscdk.services.rds.DatabaseInstanceProps props) {
        super(software.amazon.jsii.JsiiObject.InitializationMode.JSII);
        software.amazon.jsii.JsiiEngine.getInstance().createNewObject(this, new Object[] { java.util.Objects.requireNonNull(scope, "scope is required"), java.util.Objects.requireNonNull(id, "id is required"), java.util.Objects.requireNonNull(props, "props is required") });
    }

    /**
     * Import an existing database instance.
     * <p>
     * @param scope This parameter is required.
     * @param id This parameter is required.
     * @param attrs This parameter is required.
     */
    @software.amazon.jsii.Stability(software.amazon.jsii.Stability.Level.Stable)
    @OverRide
    public static @org.jetbrains.annotations.NotNull software.amazon.awscdk.services.rds.IDatabaseInstance fromDatabaseInstanceAttributes(final @org.jetbrains.annotations.NotNull software.constructs.Construct scope, final @org.jetbrains.annotations.NotNull java.lang.String id, final @org.jetbrains.annotations.NotNull software.amazon.awscdk.services.rds.DatabaseInstanceAttributes attrs) {
        return software.amazon.jsii.JsiiObject.jsiiStaticCall(software.amazon.awscdk.services.rds.DatabaseInstance.class, "fromDatabaseInstanceAttributes", software.amazon.jsii.NativeType.forClass(software.amazon.awscdk.services.rds.IDatabaseInstance.class), new Object[] { java.util.Objects.requireNonNull(scope, "scope is required"), java.util.Objects.requireNonNull(id, "id is required"), java.util.Objects.requireNonNull(attrs, "attrs is required") });
    }
```

JSII assembly also shows:

```json
"name": "fromDatabaseInstanceAttributes",
"overrides": "@aws-cdk/aws-rds.DatabaseInstanceBase",
"parameters": [
  {
    "name": "scope",
    "type": {
      "fqn": "constructs.Construct"
    }
  },
  {
    "name": "id",
    "type": {
      "primitive": "string"
    }
  },
  {
    "name": "attrs",
    "type": {
      "fqn": "@aws-cdk/aws-rds.DatabaseInstanceAttributes"
    }
  }
],
"returns": {
  "type": {
    "fqn": "@aws-cdk/aws-rds.IDatabaseInstance"
  }
},
"static": true
```

This feels like a JSII bug, it shouldn't be marking this method as an override in the assembly, and regardless, it should not generate java code that has an `@Override` annotation on a static function, better to validate and throw beforehand.

I guess this is related to Typescript quirks with inheritance of static members?

cc @RomainMuller @skinny85

Author: iliapolo

packages/@aws-cdk/aws-rds/lib/instance.ts

@@ -113,11 +113,23 @@ export interface DatabaseInstanceAttributes {
 export abstract class DatabaseInstanceBase extends Resource implements IDatabaseInstance {
   /**
    * Import an existing database instance.
-   *
-   * @deprecated use DatabaseInstance.fromDatabaseInstanceAttributes
    */
   public static fromDatabaseInstanceAttributes(scope: Construct, id: string, attrs: DatabaseInstanceAttributes): IDatabaseInstance {
-    return DatabaseInstance.fromDatabaseInstanceAttributes(scope, id, attrs);
+    class Import extends DatabaseInstanceBase implements IDatabaseInstance {
+      public readonly defaultPort = ec2.Port.tcp(attrs.port);
+      public readonly connections = new ec2.Connections({
+        securityGroups: attrs.securityGroups,
+        defaultPort: this.defaultPort,
+      });
+      public readonly instanceIdentifier = attrs.instanceIdentifier;
+      public readonly dbInstanceEndpointAddress = attrs.instanceEndpointAddress;
+      public readonly dbInstanceEndpointPort = attrs.port.toString();
+      public readonly instanceEndpoint = new Endpoint(attrs.instanceEndpointAddress, attrs.port);
+      public readonly engine = attrs.engine;
+      protected enableIamAuthentication = true;
+    }
+
+    return new Import(scope, id);
   }
 
   public abstract readonly instanceIdentifier: string;
@@ -926,27 +938,6 @@ export interface DatabaseInstanceProps extends DatabaseInstanceSourceProps {
  * @resource AWS::RDS::DBInstance
  */
 export class DatabaseInstance extends DatabaseInstanceSource implements IDatabaseInstance {
-  /**
-  * Import an existing database instance.
-  */
-  public static fromDatabaseInstanceAttributes(scope: Construct, id: string, attrs: DatabaseInstanceAttributes): IDatabaseInstance {
-    class Import extends DatabaseInstanceBase implements IDatabaseInstance {
-      public readonly defaultPort = ec2.Port.tcp(attrs.port);
-      public readonly connections = new ec2.Connections({
-        securityGroups: attrs.securityGroups,
-        defaultPort: this.defaultPort,
-      });
-      public readonly instanceIdentifier = attrs.instanceIdentifier;
-      public readonly dbInstanceEndpointAddress = attrs.instanceEndpointAddress;
-      public readonly dbInstanceEndpointPort = attrs.port.toString();
-      public readonly instanceEndpoint = new Endpoint(attrs.instanceEndpointAddress, attrs.port);
-      public readonly engine = attrs.engine;
-      protected enableIamAuthentication = true;
-    }
-
-    return new Import(scope, id);
-  }
-
   public readonly instanceIdentifier: string;
   public readonly dbInstanceEndpointAddress: string;
   public readonly dbInstanceEndpointPort: string;