Merge ORCalculator into it's only subclass

@czue Followup from https://github.com/dimagi/commcare-hq/commit/7c0fc6ec45bd11fd99e653abc20abbe51a8bc410
This is only used once, [here](https://github.com/dimagi/commcare-hq/blob/rm-or-calculator/custom/reports/mc/models.py#L405).  Not sure how to test, but the change seems innocuous enough.  What do you think?