Remove wmgRC2UDPPrefix exception for wikimania2015wiki

These are legacy exceptions, we don't want to be adding more of them
causing errors for rc patrollers about channels not existing.

It's bad enough they have to hardcoded all these exceptions, let's
not add more.

Let's revert this while we still can.

Follows-up 810edd2c494d84677bfa4d9a96f8aa60f5c6017f.

Change-Id: I27c6f82af5e9b4580686023c7f606db20e90c070