Use isQuickConfig() to determine isQuickConfig (towards php8.x fix)

[eileenmcnaughton]

Overview

Use isQuickConfig() to determine isQuickConfig

Before

Is quick config is checked twice within a few lines - once using the (preferred) function and the second time using the property. It is then called again a bit further down in the same function

After

consistent

Technical Details

With these gone the form no longer uses the (undefined) property & we don't need to super-specially set it in the price set class

Comments

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[yashodha]

@eileenmcnaughton looks good, merging this PR. Thanks!