Draft: Remove Approver and ApproverGroup model

What does this MR do and why?

After we have fully removed Approver and ApproverGroup models and their associations to Project and MergeRequest.

Now we could safely drop the tables and their data as the final step.

References

Issue - Drop approvers and approver_groups tables (#566495)

Screenshots or screen recordings

No UI changes.

How to set up and validate locally

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