mm/swapfile.c: remove the incorrect code comment

mm/swapfile.c: remove the incorrect code comment

Since commit eb085574a752 ("mm, swap: fix race between swapoff and some
swap operations"), the non_swap_entry() checking has been taken off from
function __swap_duplicate().  Hence, in the kernel-doc comment, the line
'swp_entry is migration entry -> EINVAL' is obsolete.  Remove that line to
avoid misleading people.

Link: https://lkml.kernel.org/r/20250205092721.9395-12-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>