Remove WIP support from Merge Requests

What does this MR do and why?

Removes support for prefixing an MR title with WIP (or some variation thereof)in order to toggle Draft status.

This functionality was deprecated and announced as unsupported as of %14.0 but due to engineering allocations and scheduling, we're only now getting around to actually removing it.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #330278 (closed)