oo-ui-background-image: Drop `-webkit-linear-gradient` fallback
oo-ui-background-image: Drop `-webkit-linear-gradient` fallback

This is only for unsupported browsers; we dropped this (to save on bloat)
in MediaWiki in November 2015 in T118815 (and then put back in Opera
support; see next commit).

Change-Id: I44114f4ab12075e115a389dcb7faebed1038b352

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui