rmap_remove() removes the sptep after locating the correct rmap_head but,
in several cases, the caller has already known the correct rmap_head.

This patch introduces a new pte_list_remove(); because it is known that
the spte is present (or it would not have an rmap_head), it is safe
to remove the tracking bits without any previous check.
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>