Disable UnnecessaryAnonymousClass Error Prone pattern

Is disabled by default as well.

It seems what this pattern does is suggest to replace anonymous classes
stored in constants with a corresponding method (with the intention that
the developer then uses a method reference to that method). But it even
does this when the anonymous class implements a custom functional type,
and not a generic type such as `Function`.

For example it suggested to replace
```
private static final FieldNamingStrategy CUSTOM_FIELD_NAMING_STRATEGY =
  new FieldNamingStrategy() {
    @OverRide
    public String translateName(Field f) {
      return "foo";
    }
  };
```

with a method
```
private static String customFieldNamingStrategy(Field f) {
  return "foo";
}
```

Which is probably not that helpful, and might not actually increase
readability.

pom.xml

@@ -280,7 +280,6 @@
                 -Xep:UngroupedOverloads:WARN <!-- required by style guide -->
                 -Xep:UnnecessarilyFullyQualified
                 -Xep:UnnecessarilyUsedValue
-                -Xep:UnnecessaryAnonymousClass
                 -Xep:UnnecessaryBoxedVariable:WARN
                 -Xep:UnnecessaryDefaultInEnumSwitch
                 -Xep:UnnecessaryFinal