Remove button styles from mediawiki.ui

They were split off into mediawiki.ui.button, but kept overlapping
in mediawiki.ui temporarily.  We've migrated code to use
mediawiki.ui.button, so this overlap is no longer needed.

Change-Id: I5bd23434961497d49293956c92b46ab3a6f14a41
Merge "Remove button styles from mediawiki.ui"