- Removed `system_key` from payment method model and related files.
- Updated payment method repository and services to eliminate references to `system_key`.
- Deleted the BootstrapSystemPaymentMethodsService and its use case, as they are no longer needed.
- Adjusted tests to reflect the removal of system payment methods and their associated logic.
- Added SQL script to drop the `system_key` column and its unique index from the database.
