Remove the deprecated APIs in JS envs.

In the process, the implementation of
`org.scalajs.jsenv.nodejs.JSDOMNodeJSEnv` is transferred to the
new `org.scalajs.jsenv.jsdomnodejs.JSDOMNodeJSEnv`.

js-envs/src/main/scala/org/scalajs/jsenv/ExternalJSEnv.scala

@@ -11,9 +11,7 @@ import scala.concurrent.{Future, Promise}
 import scala.util.Try
 
 abstract class ExternalJSEnv(
-    @deprecatedName('additionalArgs)
     final protected val args: Seq[String],
-    @deprecatedName('additionalEnv)
     final protected val env: Map[String, String])
     extends AsyncJSEnv {
 
@@ -27,12 +25,6 @@ abstract class ExternalJSEnv(
   /** Command to execute (on shell) for this VM */
   protected def executable: String
 
-  @deprecated("Use `args` instead.", "0.6.16")
-  final protected def additionalArgs: Seq[String] = args
-
-  @deprecated("Use `env` instead.", "0.6.16")
-  final protected def additionalEnv: Map[String, String] = env
-
   /** Custom initialization scripts. */
   protected def customInitFiles(): Seq[VirtualJSFile] = Nil
 

js-envs/src/main/scala/org/scalajs/jsenv/nodejs/AbstractNodeJSEnv.scala

@@ -23,11 +23,8 @@ import scala.concurrent.TimeoutException
 import scala.concurrent.duration._
 
 abstract class AbstractNodeJSEnv(
-    @deprecatedName('nodejsPath)
     protected val executable: String,
-    @deprecatedName('addArgs)
     args: Seq[String],
-    @deprecatedName('addEnv)
     env: Map[String, String],
     val sourceMap: Boolean)
     extends ExternalJSEnv(args, env) with ComJSEnv {

js-envs/src/main/scala/org/scalajs/jsenv/nodejs/NodeJSEnv.scala

@@ -22,22 +22,6 @@ class NodeJSEnv(config: NodeJSEnv.Config)
 
   def this() = this(NodeJSEnv.Config())
 
-  @deprecated("Use the overload with a NodeJSEnv.Config.", "0.6.18")
-  def this(
-      @deprecatedName('nodejsPath)
-      executable: String = "node",
-      @deprecatedName('addArgs)
-      args: Seq[String] = Seq.empty,
-      @deprecatedName('addEnv)
-      env: Map[String, String] = Map.empty) = {
-    this(NodeJSEnv.Config().withExecutable(executable).withArgs(args.toList).withEnv(env))
-  }
-
-  @deprecated("Use the overloaded constructor with a NodeJSEnv.Config.",
-      "0.6.18")
-  def withSourceMap(sourceMap: Boolean): NodeJSEnv =
-    new NodeJSEnv(config.withSourceMap(sourceMap))
-
   protected def vmName: String = "Node.js"
 
   override def jsRunner(files: Seq[VirtualJSFile]): JSRunner =