Cleanup feature flags and env vars for cross_database_modification

What does this MR do and why?

This MR is cleaning up feature flags and env vars that have been enabled for some time. These feature flags correspond to our "cross-database modification" detection and we want this enabled by default.

1. We enabled track_gitlab_schema_in_current_transaction on 2022-02-01. See #349944 (comment 827636394)
2. We enabled ENABLE_CROSS_DATABASE_MODIFICATION_DETECTION in production on 2021-12-08. See gitlab-com/gl-infra/k8s-workloads/gitlab-com!1390 (merged)

After we merge this and it's deployed to production we can remove ENABLE_CROSS_DATABASE_MODIFICATION_DETECTION from all our production configurations. I've created a separate issue for that #370429 (closed)

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Closes #351845 (closed)