[REF] Minor auth.net ipn simplification

[eileenmcnaughton]

Overview

[REF] Minor auth.net ipn simplification

Before

Properties set on $contribution but never used, validation not 'together'

After

Unused properties not set, validation moved closer together

Technical Details

This removes some property setting on the contribution
which is then discarded & moves a bit of validation to be
with the other validation

Comments

[civibot]

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[colemanw]

Cleanup looks solid. It certainly looks like those variables are never used.