Revert "Make PayPal total match OpenCart total"

This reverts commit 278c2df3e98ea966a37dda6e05cc04946a546acb.
Make PayPal total match OpenCart total

Resubmitted without rounding problem that caused PayPal to return an
incorrectly formatted item amount error.
As PayPal uses less precision in its calculations, its total can differ
from the cart total. Calculating subtotal this way ensures they match.