Revert "The reverse tax calculation was wrong when more than one tax applied to a given product"

This reverts commit 7a1bf53d4fe791f162966b228b93f750c364475a.

Conflicts:
	core/app/models/spree/calculator/default_tax.rb

This causes an issue where ALL tax rates of a tax category are applied, even if they do not match. This means if you have a "Clothing" tax category which has rates for EU and US, BOTH the EU and US rates will be applied.