Remove unused CSS file differential/revision-list.css
Summary:
`revision-age-fresh`, `revision-age-old`, `revision-age-stale` were added in rPb50cdc6e439c4f2915a47cb40cd94425832c22a2.
Their usage was removed in rPd9f01d6fb7f50e707805bc059aa15dcafca05160 from `src/applications/differential/view/DifferentialRevisionListView.php`.
Test Plan: Grep the code for `revision-age`. No results.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D25996