fix: remove validate_name_in_customer function (backport #41025)

[mergify]

Problem:
When we create a customer group the function "validate_name_with_customer" is called which checks whether the name of customer group exists in "Customer" DocType.
For e.g.
If we create a customer group called "XYZ", in the on_update method in customer_group.py file, the code will check whether a customer with a name of "XYZ" is already created or not. If it is created it throws an error "A customer with the same name already exists".

Solution:
Remove validate_name_with_customer from "customer_group.py" file.

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This is an automatic backport of pull request #41025 done by Mergify.