Resources.php: Remove/deprecate 'skinStyles' references to 'minerva' skin

It doesn't even exist in core: it is part of the MobileFrontend
extension. The skin style overrides should be reapplied there using
$wgResourceModuleSkinStyles: done in I30c8ce13.

OOjs UI has some weird build process that currently requires this, though.
According to James, that will be fixed "in a couple of months".

Change-Id: If82556a41c0510a70796a09b3555d2c91a7abb52
Merge "Resources.php: Remove/deprecate 'skinStyles' references to 'minerva' skin"