Resolve "Remove fill in merge request template feature"

What does this MR do and why?

We are removing all the code for the merge request template feature.

This part will remove calling the code. The second change will remove the template/services.

To Test:

• On master, create an MR and there will be a fill template button
• Switch to this branch
• Create a new MR and ensure there are no fill template button

Parts to be removed:

1. AI Actions option to use feature
2. Backend API/Prompt
3. Frontend Component
4. Documentation
5. fill_in_mr_template feature flag

Related to #463100 (closed)