Remove the attempt to cache the previous page mapped at our identity

Remove the attempt to cache the previous page mapped at our identity
location (for the wake code).  It should not be needed since we don't
map other pages at the same location and if there was an old mapping, it
would be restored by a fault.  The old code had serious problems, namely
that it was restoring the new page it had just removed (not opage) and
it could only guess at the right protection (since there's no
pmap_extract_protect function).  Thanks to Alan Cox for explaining much
of this to me.

Also, remove a commented-out initializecpu() call since it is not needed.
Restoring the cpu context is better than attempting to init from scratch.

Reviewed by:	alc (earlier version)