Revert "Make PayPal total match OpenCart total"

This reverts commit 958539976bd08d40b138cc214d078eca39060fa1.
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.