Drop the Signed-off-by requirement

Drop the Signed-off-by requirement

We've had this for roughly 10y now and it's value is dubious. Most of
xorg no longer requires, mesa accepts but doesn't require it, most of GNOME
doesn't accept it and neither does systemd.

Let's drop the requirement.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>