common/domain_page: Drop domain_mmap_cache infrastructure

common/domain_page: Drop domain_mmap_cache infrastructure

This infrastructure is used exclusively by the x86 do_mmu_update() hypercall.
Mapping and unmapping domain pages is probably not the slow part of that
function, but even with an opencoded caching implementation, Bloat-o-meter
reports:

  function                                     old     new   delta
  do_mmu_update                               6815    6573    -242

The !CONFIG_DOMAIN_PAGE stub code has a mismatch between mapping and
unmapping, which is a latent bug.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>