pmap_unmapdev()'s own direct efforts to destroy the page table entries are
redundant, so eliminate them.
Don't set PTE_W on the page table entry in pmap_kenter{,_attr}() on MIPS.
Setting PTE_W on MIPS is inconsistent with the implementation of this
function on other architectures. Moreover, PTE_W should not be set, unless
the pmap's wired mapping count is incremented, which pmap_kenter{,_attr}()
doesn't do.
MFC after: 10 days
2bc702613a