tty: remove TTY_SOFT_SAK part from __do_SAK()

Remove the TTY_SOFT_SAK part. It is never defined, so this is only
confusing.

It was actually never defined since its introduction in
0.99.14g.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118071911.12059-2-jslaby@suse.cz


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