The MOEA pmap clears PGA_WRITEABLE in pmap_remove_{all,write}(), but
fails to do so when a single mapping is removed via pmap_remove().
Correct this.
I couldn't find any other places that were missing this: this pmap does
not implement pmap_remove_pages(), and when a CoW fault removes the last
mapping of a page, its PV entry is removed using moea_remove_pvo().