[RFC][TTY] callout removal

	Folks, it had been quite a few years since callout tty devices had
been deprecated.  IMO it's time to bury them - the following

        if ((tty->driver.type == TTY_DRIVER_TYPE_SERIAL) &&
            (tty->driver.subtype == SERIAL_TYPE_CALLOUT)) {
                printk("Warning, %s opened, is a deprecated tty "
                       "callout device\n", tty_name(tty, buf));

in tty_open() had been theres since 2.1.90-pre2.  IOW, we had two stable
branches since then.

Quite a few things in tty code and related data structures exist only
because of these beasts and I'd rather get rid of them.

If nobody has serious objections I'll submit patches removing that stuff,
so if you _do_ have a reason to keep callouts around, yell now.