refactor: don't book exchange gain/loss for sales/purchase orders

[ruthra-kumar]

Don't book Exchange Gain/Loss while referencing Sales/Purchase Order

Sales/Purchase Orders don't have any accounting impact. So, Booking Gain/Loss for a Multi-Currency Order doesn't make sense.

[codecov]

Codecov Report

Merging #35061 (ce4e18c) into develop (04902e1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #35061   +/-   ##
========================================
  Coverage    63.69%   63.70%           
========================================
  Files          813      813           
  Lines        60007    60030   +23     
========================================
+ Hits         38221    38240   +19     
- Misses       21786    21790    +4     

Impacted Files	Coverage Δ
...xt/accounts/doctype/payment_entry/payment_entry.py	74.85% <100.00%> (+0.38%)	⬆️

... and 15 files with indirect coverage changes

[deepeshgarg007]

See similar logic being used for the exchange rate in both places, maybe move this to a function and use that?

Also if we can add a simple unit test for this