Stop always loading MonoBook and Vector

Removing the hack added in Ib4bdda5e.

This will cause an error message to be shown to almost every MediaWiki
user who upgrades their installation (including us developers) until
they add entries for their skins to LocalSettings. This is deemed an
acceptable trade-off, and the message makes it easy to resolve the
issue.

Bug: 68402
Change-Id: I2596ef73088ce94d78ce3dc3ae4da9d81023a2cb