This was to counteract hardware that doesn't work well out of the box,
resulting in quite different behavior across devices. Specifically, only
some trackpoints even have the sensitivity setting.

Change to take over all of the pointer acceleration on trackpoints, so we can
control the actual behavior mostly independent of the system setting. So we
drop the CONST_ACCEL parsing (which never was handled as const accel anyway)
and undo the effect that the SENSITIVITY udev property has. [1]

We take a default range at the default sensitivity and multiply it by the
proportion of the current sensitivity. This seems to be accurate enough.

[1] In the future, we should read not only the property but also the sysfs file to
make sure we're handling the right value, but for now this will do.

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