Removes status checks add status field feature flag

Removes status_checks_add_status_field flag

Screenshots or screen recordings

	Screenshot
All pending
All passed
Mixed statuses

How to set up and validate locally

• Create a new project.
• Create three external status checks for that project. https://docs.gitlab.com/ee/user/project/merge_requests/status_checks.html#add-or-update-a-status-check
• Create a new branch, make a small change, and create a merge request on that project.
• In the MR widget, validate that all 3 status checks are pending.
• Approve one, fail one, and do nothing to the other. (API docs)
• Assert that the widget looks as expected.

Related to #353836 (closed)