mm/swapfile.c: remove the unneeded checking

In free_swap_and_cache_nr(), invocation of get_swap_device() has done the
checking if it's a swap entry.  So remove the redundant checking here.

Link: https://lkml.kernel.org/r/20250205092721.9395-10-bhe@redhat.com


Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <ryncsn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>