Remove deployment_merge_requests feature flag

In !18755 (merged) we added the ability to track merge requests deployed using the GitLab environments and deployments feature. This has been using a feature flag so we could test it on a small scale, before making it available to everybody.

Since the changes are working without causing any trouble, we should remove this feature flag and track this data by default for every project. We should maybe also reword the changelog entry a bit so its not as cryptic as "Add deployment_merge_requests table".