Remove fill in merge request template feature

Details

We've made the decision to deprecate and remove the Fill in merge request template feature. Items 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
6. Entry in the left nav, if any
7. Pricing / marketing pages
   1. E.g.: https://about.gitlab.com/gitlab-duo/#pricing