refactor: removed unrequired code and test for standalone delivery note serial return

[noahjacob]

Changes

• Delivery Note does not allow a user to create a standalone return, so removed the test for it.
• Since a return can only be created via a doc, it doesn't need to call update_serial_nos_after_submit since this is only needed for cases where serials are auto-created.

[codecov]

Codecov Report

Merging #30276 (b01bdf1) into develop (b93ce78) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop   #30276      +/-   ##
===========================================
- Coverage    60.51%   60.45%   -0.07%     
===========================================
  Files         1084     1084              
  Lines        68611    68609       -2     
===========================================
- Hits         41522    41477      -45     
- Misses       27089    27132      +43     

Impacted Files	Coverage Δ
...pnext/stock/doctype/delivery_note/delivery_note.py	66.57% <100.00%> (-0.19%)	⬇️
...em_wise_sales_register/item_wise_sales_register.py	51.36% <0.00%> (-11.37%)	⬇️
...t/accounts/report/sales_register/sales_register.py	71.51% <0.00%> (-7.28%)	⬇️
erpnext/hr/doctype/job_offer/job_offer.py	82.92% <0.00%> (-4.88%)	⬇️
...pnext/setup/doctype/sales_partner/sales_partner.py	65.21% <0.00%> (-4.35%)	⬇️
...ctype/woocommerce_settings/woocommerce_settings.py	80.76% <0.00%> (-3.85%)	⬇️
...pnext/accounts/report/gross_profit/gross_profit.py	81.81% <0.00%> (-3.72%)	⬇️
...e/asset_value_adjustment/asset_value_adjustment.py	86.04% <0.00%> (-3.49%)	⬇️
erpnext/education/doctype/student/student.py	73.68% <0.00%> (-3.16%)	⬇️
erpnext/stock/doctype/warehouse/warehouse.py	45.83% <0.00%> (-2.50%)	⬇️
... and 22 more