TTY: isdn, remove callout

I wonder how this survived there during the whole 2.6 series until now
:D.

Callouts are not used for a decade, so let us remove it also from
isdn. This means removal of ISDN_ASYNC_CALLOUT_ACTIVE which is never
raised in info->flags and callout_termios which are never used.

This will help us to get rid of ISDN_ASYNC_* flags and use ASYNC ones
from serial.h. And then we will switch to tty_port.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>