Cleanup on Apache Twill code.

When doing previous PR I saw some small cleanups opportunity:
1. Remove unnecessary public meodifiers in the interfaces method definition because they automatically public.
2. All field members declared in an interface are by default public, static and final so it is redundant to say it again.
3. Remove unused imports.
4. Remove private modifier in enum constructors bc they inherently private.
5. Remove static modifier for inner classes for interface bc they inherently static.

This closes #53 on GitHub.

Signed-off-by: Terence Yim <chtyim@apache.org>
8 files changed