net/af_unix: Remove unused old_pid variable

Commit 109f6e39fa ("af_unix: Allow SO_PEERCRED
to work across namespaces.") introduced the old_pid variable
in unix_listen, but it's never used.
Remove the declaration and the call to put_pid.

Signed-off-by: Or Cohen <orcohen@paloaltonetworks.com>
Link: https://lore.kernel.org/r/20201011153527.18628-1-orcohen@paloaltonetworks.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/af_unix.c

@@ -613,7 +613,6 @@ static int unix_listen(struct socket *sock, int backlog)
 	int err;
 	struct sock *sk = sock->sk;
 	struct unix_sock *u = unix_sk(sk);
-	struct pid *old_pid = NULL;
 
 	err = -EOPNOTSUPP;
 	if (sock->type != SOCK_STREAM && sock->type != SOCK_SEQPACKET)
@@ -634,7 +633,6 @@ static int unix_listen(struct socket *sock, int backlog)
 
 out_unlock:
 	unix_state_unlock(sk);
-	put_pid(old_pid);
 out:
 	return err;
 }