Remove unused imports from what-changed.

Testing Done:
/pants test tests/python/pants_test/tasks:what_changed

https://travis-ci.org/pantsbuild/pants/builds/45154573

Bugs closed: 890

Reviewed at https://rbcommons.com/s/twitter/r/1542/
Sync of https://github.com/pantsbuild/pants/commit/d0a34305012d1ad91fddddb2060abb87a50af667