tty: drop outdated comments about release_tty() locking

The current version of the TTY code unlocks the tty_struct(s) before
release_tty() rather than after.  Moreover, tty_unlock_pair() no longer
exists.  Thus, remove the outdated comments regarding tty_unlock_pair().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20200224073359.292795-1-ebiggers@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -1589,9 +1589,7 @@ void tty_kclose(struct tty_struct *tty)
 	tty_debug_hangup(tty, "freeing structure\n");
 	/*
 	 * The release_tty function takes care of the details of clearing
-	 * the slots and preserving the termios structure. The tty_unlock_pair
-	 * should be safe as we keep a kref while the tty is locked (so the
-	 * unlock never unlocks a freed tty).
+	 * the slots and preserving the termios structure.
 	 */
 	mutex_lock(&tty_mutex);
 	tty_port_set_kopened(tty->port, 0);
@@ -1621,9 +1619,7 @@ void tty_release_struct(struct tty_struct *tty, int idx)
 	tty_debug_hangup(tty, "freeing structure\n");
 	/*
 	 * The release_tty function takes care of the details of clearing
-	 * the slots and preserving the termios structure. The tty_unlock_pair
-	 * should be safe as we keep a kref while the tty is locked (so the
-	 * unlock never unlocks a freed tty).
+	 * the slots and preserving the termios structure.
 	 */
 	mutex_lock(&tty_mutex);
 	release_tty(tty, idx);