Delete unused custom bug_activity.css file

The file includes two definitions ("bz_bugactivity").
No results when grepping for them in our custom changes, in
Bugzilla upstream trunk, and Bugzilla upstream 4.2.

No bug reports mentioning bz_bugactivity or bug_activity.css
either in bugzilla.mozilla.org.

Bug: 54823
Change-Id: I0fd3b476489810470368b3561e91b87d1c466421