arm64: remove an unneeded test from pmap_clear_modify()

The page table entry for a 4KB page mapping must be valid if a PV entry
for the mapping exists, so there is no point in testing each page table
entry's validity when iterating over a PV list.

Reviewed by: kib, markj

(cherry picked from commit 62ea198e95f139e6b8041ec44f75d65aa26970d0)