tty: pty, remove BUG_ON from pty_close

tty->ops->close is always called with a valid tty, so the BUG_ON cannot
trigger.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210105120239.28031-5-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>