[REF] Move use of priceSetID & amount_override to where they are used

[eileenmcnaughton]

Overview

Minor code cleanup

Before

$priceFields & $amount_override defined at the start of the fn - giving the impression they are generic in the function

After

$priceFields removed as not necessary, amount_override moved to where it is used. (note that this is all cheap variable parsing)

Technical Details

The field name is misleading since it implies it is useful in general to pass in a priceSetID - however, it has
a very specific usage & meaning & ideally we would deprecate it in favour of something more readable. For now
group the code more logically & comment it

Comments

[civibot]

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[mattwire]

Two things. In theory this reduces performance slightly since we'll now be running this code for every checkbox instead of once per priceset. Also amount_override will be reset to NULL each run (if there's a checkbox) and potentially set to a different value. In practise the performance issue is likely to be trivial and amount_override will always evaluate the same variables.
From experience this code is really hard to work with and cleanup like this really helps later on

[eileenmcnaughton]

@mattwire the overwrite thing is OK because amount Override is only valid if there is only one line item - so only one iteration. The performance is not doing any queries so should be negligable