Remove legacy flag support from new options.

We no longer need it now that we're on the new parser.

Removed a couple of legacy= kwargs, to prove that things still work.
Future changes will clean up the remaining kwargs from option registrations.

Testing Done:
Integration and unit tests passed.

Full CI baking.

Reviewed at https://rbcommons.com/s/twitter/r/1328/
Sync of https://github.com/pantsbuild/pants/commit/7c4db109a4d42b1a41b39bd12efe38c1c574a946