af_unix: Remove unix_unhash()

Dummy unix_unhash() was introduced for sockmap in commit 94531cfc
("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
d359902d ("af_unix: Fix NULL pointer bug in unix_shutdown"), and in BPF
proto's sock_map_unhash() since commit 5b4a79ba ("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>