Reduce the size of a PV entry by eliminating pv_ptem.  There is no need
to store a pointer to the page table page in the PV entry because it is
easily computed during the walk down the page table.

Eliminate the ptphint from the pmap.  Long, long ago, page table pages
belonged to a vm object, and we would look up page table pages based
upon their offset within this vm object.  In those days, this hint may
have had tangible benefits.