Clean up custom show_bug.css

Remove all definitions which are a 1:1 copy in upstream (as we
fall back to upstream CSS anyway for items not defined in our
custom CSS). This simplifies maintenance of "real" changes
that we have.

Bug: 54823
Change-Id: Idc2b399391ff20b6308e74cf37f1c927f54699a4
Merge "Clean up custom show_bug.css"