fix: Correct price list rate field value in return Sales Invoice

[nemesis189]

Problem:
Submit a sales invoice with a different price list. Create a return credit for this submitted invoice and on expanding the item row in the items table the price list rate field will display 0 instead of the actual price list rate.

Solution:
the update_item method was setting the field to 0 in an earlier PR, removed that part.

Steps:

1. Create a new price list rate
2. Create an item price list rate under the new price list
3. Create a Sales Invoice with this price list applied and submit.
4. From the submitted invoice, create a return credit note and expand the item row and check the price list rate field.

[coveralls]

Coverage decreased (-0.05%) to 51.322% when pulling 76021ff on nemesis189:return-si-price-list-rate-field-fix into d4d5a42 on frappe:develop.