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
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30875