tty: return void from tty_unregister_ldisc

Now that noone checks the return value of tty_unregister_ldisc, make the
function return 'void'.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210505091928.22010-20-jslaby@suse.cz


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