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>