Eliminate a redundant pmap_load() from pmap_remove_pages().

There is no reason why the pmap_invalidate_all() in pmap_remove_pages()
must be performed before the final PV list lock release. Move it past
the lock release.

Eliminate a stale comment from pmap_page_test_mappings(). We implemented
a modified bit in r350004.

MFC after: 1 week