Clean up FF deprecate_approver_and_approver_group

What does this MR do and why?

Clean up FF deprecate_approver_and_approver_group

After we introduced FF and have rolled out 100%, now we plan to clean up FF.

• Left presenter with method, and return empty array to reduce potential reference risk.
• Leave Grape Entity definition, and always return empty array.

References

Issue - [FF] `deprecate_approver_and_approver_group` --... (#573012)

Screenshots or screen recordings

How to set up and validate locally

• Find one project from local Rails console, and then create its approver and approver_group (should be simple, just some associations).
• Use master branch
• Get GDK running locally
• ::Feature.enable(:deprecate_approver_and_approver_group) in Rails console
• Curl curl -H "PRIVATE-TOKEN: YOUR-TOKEN" "http://gdk.test:3000/api/v4/projects/28/approvals" and see approvers and approver_groups are returned as expected
• Now switch to this branch
• Curl curl -H "PRIVATE-TOKEN: YOUR-TOKEN" "http://gdk.test:3000/api/v4/projects/28/approvals"
• And see NO approvers and approver_groups are returned

MR acceptance checklist

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