af_unix: Remove U_LOCK_GC_LISTENER.

Commit 1971d13f ("af_unix: Suppress false-positive lockdep splat for
spin_lock() in __unix_gc().") added U_LOCK_GC_LISTENER for the old GC,
but it's no longer needed for the new GC.

Let's remove U_LOCK_GC_LISTENER and unix_state_lock_nested() as there's
no user.
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/net/af_unix.h

@@ -96,18 +96,6 @@ struct unix_sock {
 
 #define unix_state_lock(s)	spin_lock(&unix_sk(s)->lock)
 #define unix_state_unlock(s)	spin_unlock(&unix_sk(s)->lock)
-enum unix_socket_lock_class {
-	U_LOCK_NORMAL,
-	U_LOCK_GC_LISTENER, /* used for listening socket while determining gc
-			     * candidates to close a small race window.
-			     */
-};
-
-static inline void unix_state_lock_nested(struct sock *sk,
-				   enum unix_socket_lock_class subclass)
-{
-	spin_lock_nested(&unix_sk(sk)->lock, subclass);
-}
 
 #define peer_wait peer_wq.wait