Cleanup recordAdditionalPayment towards #11556

[mattwire]

Overview

Minor code tidy up, rename variable for clarity

Before

Code less easy to understand

After

Code easier to understand

Technical Details

This makes it clear that updateMembershipBasedOnCompletionOfContribution is only called from recordAdditionalPayment when contribution status = Completed

@eileenmcnaughton towards #11556

[civibot]

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[eileenmcnaughton]

I checked this change out & is sensible & correct