parisc: Drop pa_swapper_pg_lock spinlock

This spinlock was dropped with commit b7795074 ("parisc: Optimize
per-pagetable spinlocks") in kernel v5.12.

Remove it to silence a sparse warning.

Signed-off-by: Helge Deller <deller@gmx.de>
Reported-by: kernel test robot <lkp@intel.com>
Cc: <stable@vger.kernel.org> # v5.12+