Remove @hide javadoc from generated classes (firebase#1871)

It's not needed for the documentation purposes as javadoc and dokka only run on source, not generated code.

Author: vkryachko

encoders/firebase-encoders-processor/src/main/java/com/google/firebase/encoders/processor/EncodableProcessor.java

@@ -97,7 +97,6 @@ private void processClass(Element element) {
  // before and after 9. See https://github.com/google/dagger/pull/882
  TypeSpec.Builder encoderBuilder =
  TypeSpec.classBuilder(className)
- .addJavadoc("@hide")
  .addModifiers(Modifier.PUBLIC, Modifier.FINAL)
  .addSuperinterface(configurator)
  .addField(