no longer git-ignore these no-longer-generated files

We stopped generating these back in July. Surely everybody's done a 'git clean' since then. Surely.

Testing Done:
git status

Bugs closed: 371

Reviewed at https://rbcommons.com/s/twitter/r/1144/
Sync of https://github.com/pantsbuild/pants/commit/4ddef92b62699b513ab7fe8c3006d13c92823a89