Remove prevent_modifications_of_mr_rules_post_merge flag

What does this MR do and why?

For compliance, after merging, we need to prevent changes to the approval state of a Merge Request.

This merge request is to remove the feature flag that prevents changes. The feature flag has been turned on for 1 and a half weeks.

Feature flag rollout Issue: gitlab-com/gl-infra/production#17331 (closed)

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