hwdb: drop POINTINGSTICK_CONST_ACCEL
The TODO says we were supposed to do that in 2019–2020 (if I interpreted the
enigmatic notation correctly). The comment in hwdb said:
> DO NOT USE THIS PROPERTY. This property is kept for backwards
> compatibility. The only known consumer, libinput, stopped reading this
> property in version 1.9.0. No new entries for this property should be
> added.
… and we're currently on libinput-1.23.0.
Most likely there are no users, and even if they are, they'll just get a
slightly misbehaving pointingstick, which shouldn't be too bad.