Clean up Azure Mysql Single Server related code

[Azure Mysql Single Server Code is deprecated](https://learn.microsoft.com/en-us/azure/mysql/migrate/whats-happening-to-mysql-single-server) and from March 28 2025 it will be completely decommissioned. Therefore there is no point of having this code in our codebase, and it's even unclear if calling the mysql single endpoint won't even cause errors in cloudbeat.

[The Integration Test is already disabled.](https://github.com/elastic/cloudbeat/blob/main/tests/product/tests/data/azure/azure_database_service_test_cases.py#L26).

What to clean:
- [Integration Tests](https://github.com/elastic/cloudbeat/blob/main/tests/product/tests/data/azure/azure_database_service_test_cases.py#L26)
- Anything related to [`AzureMySQLDBType = "azure-mysql-server-db"`](https://github.com/elastic/cloudbeat/blob/main/internal/resources/fetching/fetcher.go#L84)
- [Clean rule 4.4.1](https://github.com/romulets/cloudbeat/tree/0490f0161a8bb50ead3a72cdda792dd6e58dc5c4/security-policies/bundle/compliance/cis_azure/rules/cis_4_4_1)
- Before cleaning, verify if indeed we should just clean it or something is needed to do with flexible servers.
- Clean up any related OPA helper function
- [PostgreSQL tests cleanup](https://github.com/elastic/cloudbeat/pull/2600)