Completely remove the $wgMFCustomLogos['logo'] stuff

It's been broken for a long time and doesn't comply with our current
designs anyway. Remove mw.png and don't default the mobile logo to it:
it's too small for our current usage of mobile logo and it's kinda pointless
to use a MW logo there.

Change-Id: I5501842553334e93cb67f213df7bc898fd7e2802
Merge "Completely remove the $wgMFCustomLogos['logo'] stuff"