Drop mediawiki.skinning.interface in preparation for its deprecation
Since MonoBook is never cached on Wikimedia wikis, we don't need
to do anything about both modules being loaded in cached HTML.
There should be no differences before and after this change.
The change must apply to skins.monobook.styles and skins.monobook.responsive
as these modules never load at the same time (see setupSkinUserCss).