make frwiki thumbs the default again

Having different thumbnails sizes per wiki is not something we can
properly support right now.  Our cache and files infrastructure wîll end
being filled with thumbs which are barely used beside on the french
wiki.

The frwiki request was bug 27839

We had a similar request for hewiki which has been rejected, have a look
at bug 41712.

This patch reverts aa433140c

Change-Id: Id480d4d9295f6b496cf61b31df589bcdd1456cc1