remove some checks for impossible conditions.
in pmap_enter(), only call pmap_remove() to remove an existing mapping
if there actually is an existing mapping.
in pmap_remove_pv(), don't flush the MIPS1 cache when removing the last mapping.
this was added in rev 1.97, to avoid stale data being left in the cache
when the page is zeroed bypassing the cache in pmap_zero_page_uncached().
we've since found that bypassing the cache for idle-loop page zeroing
doesn't work very well anyway, so we don't do that anymore.
so now we can remove the extra cache-flush.
remove pmap_zero_page_uncached() while I'm thinking of it.
various other cleanup.
873e926c7b