Previously we had two different sets of tax updating code.  One was
Order#create_tax_charge! and the other was the code in OrderUpdater.
The effect of the former encompasses the effect of the latter, so
functionally we can merge these.

This may result in increased load since we're now using the more
encompassing method which was more heavyweight.

We had several specs that manually created tax adjustments rather than
setting up a real tax rate to be applied to the order.  This fixes
that.

To allow easier customization of tax calculation in extensions or
applications.