fix(pm/wiki): remove local interwiki prefixes

This option specifies what other wikis call this one - in our case
it's FBC (for the wiki on teal) and S (for the wiki on umber). We don't
really need this for our use, but we *do* need to be able to refer to
the other wiki for interwiki links...

We may want to make this an option later, but it certainly isn't correct
to have it the same on every wiki...