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

This support for pre-v.15 Opera uses increases the payload and
processing cost of every icon for all users to serve about 0.15%
of users.

Bug: T121517
Change-Id: Ib31ed695409fe14a7340a5412cd9885b06296ff3

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