Remove hack to add java_sources in context

Testing Done:
Yes:
https://travis-ci.org/pantsbuild/pants/builds/37349364

Maunally generated idea for
./pants goal idea testprojects/src/scala/com/pants/testproject/javasources:javasources and
./pants goal idea testprojects/src/scala/com/pants/testproject/javasources:javasources --idea-intransitive

Both have dependent java_sources targets configured in generated Project Structure.

With Eric's hack to add java_sources in ide_gen for scala_library, we no longer need the java_sources to be added to the self.context targets.
https://github.com/pantsbuild/pants/blob/master/src/python/pants/backend/jvm/tasks/ide_gen.py#L552

Reviewed at https://rbcommons.com/s/twitter/r/1130/
Sync of https://github.com/pantsbuild/pants/commit/7e48e0aad8d93f47e78a76dcb65c929e36020801