libinput: do not switch output associations on disable

If an input device is associated to an output that then gets disabled,
there is no case where associating to a different output would be
correct.

The output association is used for absolute positioned input devices,
and an input device like a touchscreen cannot ever be automatically
valid for more than one possible output - the touchscreen display
device.

Therefore do not automatically reassing implicitly associated input
devices to another output. This removes some log spam on shutdown.

In fact, if there can be more than one output at any time, absolute
input devices must be explicitly configured to associate with the
correct output, or the results are essentially undefined in any case.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>