Remove the `AllChecksSuccessful` check and the auto-merge capability

[premun]

Context

As discussed here - https://github.com/dotnet/arcade-services/pull/4026 - Maestro has the following problem with GitHub checks:

If a check is skipped, the AllChecksSuccessful check will fail unless that particular check is ignored completely in Maestro's branch policies. However, the check is reported as "neutral" by GitHub and not skipped, and we cannot ignore neutral results because checks are also neutral for a brief time right after they get created and before Azure DevOps picks them up and starts a build. This means that a PR is might be considered as successful right after creation and get merged which goes against our policies.

Goal

The goal might be to remove AllChecksSuccessful altogether (or make it do nothing) and remove all the auto-merge capability.