[REFACTOR] Remove `payment_method_id` being used as a fallback for `locker_id`

[Chethan-rao]

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.