Undeploy the skins, SkinPerPage and FormPreloadPostCache extensions

These skins are terrible, unused (at least nobody has them as default
on foundationwiki) and mostly broken, that's why I see no use in
having this extension deployed.

The SkinPerPage extension doesn't work anymore and is only being
used on old Donation landing pages (from 2007 and 2008).

The FormPreloadPostCache is barely useful and makes use of some
black magic in order to perform its task. Also it's no longer
needed as donations go through donatewiki now.

Change-Id: I5c2d201552cfe5ab4d974f8644623b0b5b6b431b