Try to remove QQuickVelocityCalculator

Theoretically it should be obsolete, because we have QEventPoint::velocity now. But we haven't been using QEventPoint::velocity much yet, so it remains to be seen if that would introduce any new bugs; so this has a bit of risk and maybe should be done at the beginning of a non-LTS dev cycle (e.g. 6.4 or so).

OTOH as part of the goal of using pointer handlers more, in Controls, maybe we won't need to worry about velocity directly anymore, but rather make velocity-sensitive pointer handler features as needed.