Deps attribute was unused. Sources set is also never used. The idiom is
to use rollup_bundle() rule first and then pass in the result into
polygerrit_plugin rule as app attribute.
I verified with this query: [1], that in fact sources set is never used.
[1] https://cs.bazel.build/search?q=r%3Agerrit+polygerrit_plugin
Change-Id: Ie46a8f02a2b485a856916fe14002dc6d0e710c20