Clear PGA_WRITEABLE in moea_pvo_remove().

moea_pvo_remove() might remove the last mapping of a page, in which case
it is clearly no longer writeable. This can happen via pmap_remove(),
or when a CoW fault removes the last mapping of the old page.

Reported and tested by: bdragon
Reviewed by: alc, bdragon, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22044