af_unix: Remove unix_unhash()

Dummy unix_unhash() was introduced for sockmap in commit 94531cfcbe
("af_unix: Add unix_stream_proto for sockmap"), but there's no need to
implement it anymore.

->unhash() is only called conditionally: in unix_shutdown() since commit
d359902d5c ("af_unix: Fix NULL pointer bug in unix_shutdown"), and in BPF
proto's sock_map_unhash() since commit 5b4a79ba65 ("bpf, sockmap: Don't
let sock_map_{close,destroy,unhash} call itself").

Remove it.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250409-cleanup-drop-unix-unhash-v1-1-1659e5b8ee84@rbox.co
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/af_unix.c

@@ -950,13 +950,6 @@ static void unix_close(struct sock *sk, long timeout)
 	 */
 }
 
-static void unix_unhash(struct sock *sk)
-{
-	/* Nothing to do here, unix socket does not need a ->unhash().
-	 * This is merely for sockmap.
-	 */
-}
-
 static bool unix_bpf_bypass_getsockopt(int level, int optname)
 {
 	if (level == SOL_SOCKET) {
@@ -987,7 +980,6 @@ struct proto unix_stream_proto = {
 	.owner			= THIS_MODULE,
 	.obj_size		= sizeof(struct unix_sock),
 	.close			= unix_close,
-	.unhash			= unix_unhash,
 	.bpf_bypass_getsockopt	= unix_bpf_bypass_getsockopt,
 #ifdef CONFIG_BPF_SYSCALL
 	.psock_update_sk_prot	= unix_stream_bpf_update_proto,