uml: remove unused fields from mm_context

The 3-level page table fixes forgot to remove a couple now-unused fields from
struct mm_context.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>