tty: Remove warning in tty_lock_slave()

Commit 2aff5e2bc6 ('tty: Change
tty lock order to master->slave') added a warning which is broken
and unnecessary now that the tty lock has fixed lock subclasses,
added in commit 2febdb632b
('tty: Preset lock subclass for nested tty locks').

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_mutex.c

@@ -46,12 +46,8 @@ EXPORT_SYMBOL(tty_unlock);
 
 void __lockfunc tty_lock_slave(struct tty_struct *tty)
 {
-	if (tty && tty != tty->link) {
-		WARN_ON(!mutex_is_locked(&tty->link->legacy_mutex) ||
-			!tty->driver->type == TTY_DRIVER_TYPE_PTY ||
-			!tty->driver->type == PTY_TYPE_SLAVE);
+	if (tty && tty != tty->link)
 		tty_lock(tty);
-	}
 }
 
 void __lockfunc tty_unlock_slave(struct tty_struct *tty)