Revert "Expose a custom acceleration profile"

Revert "Expose a custom acceleration profile"

This looked good on paper but clearly no-one (including myself) ever tested this
in a real-life situation or they would've noticed that the constant factor is
missing, causing a segfault on the first two-finger scroll event, touchpad
gesture or button scrolling.

Adding the constant factor makes the API much worse and the benefit is
unclear, so out of the window it goes. We can revisit this for libinput 1.12
but this isn't going to make the next release.

This reverts commit d8bd650540e68e8b648e76180c5eee0f19a3b893.

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