Remove wgSkipSkin and wgSkipSkins

I've been around for a while, and seen quite a lot in that time, but I
have never heard of skins named "htmldump", "wikimania", or especially
"drsport". And I just found out that monobookcbt was deleted in 2009. So
I really don't think this code is doing anything anymore.

Change-Id: I551ecc4136ee920d3e015f69a613092ddb7c973e