Decommision getPartialPaymentTrxn function

Decommision getPartialPaymentTrxn function

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