mm/swap_state.c: remove the meaningless code comment

Since commit 8d93b41c ("mm: Convert add_to_swap_cache to XArray"),
there's no returned _EEXIT, so the code comment doesn't make sense any
more.

Link: https://lkml.kernel.org/r/20250205092721.9395-9-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>