Decommision getPartialPaymentTrxn function

[eileenmcnaughton]

Overview

Readability fix - remove internal function

Before

getPartialPaymentTrxn called in order to do updates

After

Misleading function removed

Technical Details

This function is only called from one place in the code & also from a test.

It is misleading as it implies a 'get' when it actually does an update
and it groups functionality in a way that doesn't make sense from the calling code pov

In this commit the function is removed and the lines are directly copied to the calling
function, as preparation for cleaning up the calling function

Comments

@monishdeb this is reduced from #13690 - just moving code lines - I realise now there is some more scariness in the handling in that fn - test cover has been really good!

[civibot]

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[monishdeb]

Ok agree with this change. Only occurence of CRM_Core_BAO_FinancialTrxn::getPartialPaymentTrxn is only on the Payment class in entrire code-base and is safely shipped inside Payment.create BAO fn. Existing UT is correctly modified and most importantly (pause)

PASSED

Merging now.