Always enable the database load balancer

What does this MR do?

This always enables the database load balancing code, even when no replicas are configured. This reduces the amount of different code paths, and in turn makes it easier to add support for multiple databases (e.g. because we don't need to check if the LB is enabled all over the place).

See #338053 (closed) for more information

TODO

• Double check if GitLab works without replicas being configured
• Double check that migrations still work as expected (= no replicas are used)
• Double check replicas are honoured when configured
• Double check that service discovery still works

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.