tty: remove tty NULL check from __do_SAK()

Both do_SAK_work() and vc_SAK() provide a valid tty to __do_SAK(), so
remove the check.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118071911.12059-3-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -3033,9 +3033,6 @@ void __do_SAK(struct tty_struct *tty)
 	int		i;
 	unsigned long flags;
 
-	if (!tty)
-		return;
-
 	spin_lock_irqsave(&tty->ctrl.lock, flags);
 	session = get_pid(tty->ctrl.session);
 	spin_unlock_irqrestore(&tty->ctrl.lock, flags);