eliminate special FLUSH flag from page table

This was used to flush a region even if the page table entry had been
cleared.  In theory this was never necessary, but now we've switched to
alias based flushing, the whole set of code associated with it can be dumped.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>