Remove the code that switches around MMU domains on armv7. MMU domains are

Remove the code that switches around MMU domains on armv7.  MMU domains are
basically a relic from the past.  Using them doesn't make a lot of sense
the way our pmaps work.  Support for MMU domains isn't present in
long-descriptor translation table format, so it is clearly on its way out.

Based on a diff from Artituri Alm.

ok patrick@