[REF] Separate and move line-item specific portion of checkTaxAmount to LineItem

[REF] Separate and move line-item specific portion of checkTaxAmount to LineItem

This is not intended to be the final place but making sense of checkTaxAmount is hard so I wanted to keep this simple and
iterate.

checkTaxAmount is called from 2 places - once from v3 LineItem.create with isLineItem = TRUE
and once from Contribution.create with isLineItem = FALSE

Most of the function is accessible dependent on isLineItem - meaning it's 2 separate functions.
However, the first chuck is called 'whenever id is set'. BUT it makes no sense for line items. It
uses the 'id' - regardless of whether it is a line item id or a contribution id - to get the previous
contribution. It then sets tax amount based on a comparison - but
that really makes zero sense. I'm pretty sure it was never intended to be used for LineItems