io_uring/msg_ring: Drop custom destructor

kfree can handle slab objects nowadays. Drop the extra callback and just
use kfree.

Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://lore.kernel.org/r/20241216204615.759089-10-krisman@suse.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>