arm: Avoid redundant clearing of the PGA_REFERENCED flag after pmap_nuke_pv()

On pv_entry removal in pmap_nuke_pv(), all corresponding  pv_entries related to
specified vm_page are being checked for reference and PGA_REFERENCED flag is
cleared if there are no referenced mappings or no pv_entries left.
Hence, additional PGA_REFERENCED flag clearing is not necessary.
arm: Use pmap_find_pv if needed instead of multiplying its code throughout pmap.