Adapt tlb flush calls following arm64/pmap.c

Adapt tlb flush calls following arm64/pmap.c

1. in pmap_enter() no need to call tlb_flush_page() if we don't actually
insert a pted
2. all callers of pmap_pte_remove() already call tlb_flush_page()

This seems to result in some performance improvement (18mn -> 17mn15)
while building libc on a Hifive Unmatched).  Also zap whitespace and
useless comments to further reduce the diff with arm64/pmap.c

ok kettenis@