TTY: serial, remove dead code from 68328

The code is dead at least since 2002. So remove it to not distort git
grep output (about port.tty usage).

Remove the whole do_softirq tasklet as it's noop now.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>