Remove `(?:in|de)crement!?`

These methods are useless sugar and are quite dangerous. They implement
last-write-wins while having names that suggest atomicity and safety.

The AR convention seems to prefer that `UPDATE` queries be made via
class methods - these methods are already available in `CounterCache`.

Reintroducing these methods is a tricky proposition, because doing it
right means adding a read immediately after the update to update the
in-memory value; this requires a transaction and some state tracking to
ensure that other dirty attributes are not discarded.

This might be appropriate for 5.0.

cc @bogdan