Remove unused release-notes.css

The file was renamed to page.css in upstream years ago. Hence this
custom version is not loaded anymore anyway and can die.
See https://bugzilla.mozilla.org/show_bug.cgi?id=518293#c19

Bug: 54823
Change-Id: I85f1341c100d27e002ad62a5ce0cb948bb675632
Merge "Remove unused release-notes.css"