refactor(payment_methods): remove `payment_method_id` being used as a fallback for `locker_id`

[Chethan-rao]

Type of Change

• [ ] Bugfix
• [ ] New feature
• [ ] Enhancement
• [x] Refactoring
• [ ] Dependency updates
• [ ] Documentation
• [ ] CI/CD

Description

Currently, if locker_id was not found in the payment_methods entry, we used to assume payment_method_id as locker_id. But since all the migrations have been taken care of, it is safe to remove this fallback logic. locker_id itself is an optional field in the application. So, if it is not found in case of card and bank_transfer, we should throw an error.

Additional Changes

• [ ] This PR modifies the API contract
• [ ] This PR modifies the database schema
• [ ] This PR modifies application configuration/environment variables

Motivation and Context

How did you test it?

Checklist

• [x] I formatted the code cargo +nightly fmt --all
• [x] I addressed lints thrown by cargo clippy
• [x] I reviewed the submitted code
• [ ] I added unit tests for my changes where possible
• [ ] I added a CHANGELOG entry if applicable