Delete the add_ai_summary_for_new_mr feature flag

This feature flag was introduced in 16.9, which is more than 4 milestones ago.

As part of our process we want to ensure feature flags don't stay too long in the codebase.

Rollout issue: gitlab-com/gl-infra/production#17533

Remaining mentions of the feature flag (click to expand)

doc-locale/ja-jp/api/graphql/reference/_index.md
45974:2つのブランチに基づいて新しいマージリクエストを要約します。`null`機能フラグが無効になっている場合、`add_ai_summary_for_new_mr`を返します。

doc/api/graphql/reference/_index.md
53917:Summarize a new merge request based on two branches. Returns `null` if the `add_ai_summary_for_new_mr` feature flag is disabled.

doc/user/project/merge_requests/duo_in_merge_requests.md
32:- Feature flag `add_ai_summary_for_new_mr` [enabled by default](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/186108) in GitLab 17.11.
35:- [Generally available](https://gitlab.com/gitlab-com/gl-infra/production/-/issues/17533) in GitLab 18.6. Feature flag `add_ai_summary_for_new_mr` removed.

ee/spec/support/shared_examples/ai/model_selection_feature_setting_shared_examples.rb
174:          review_merge_request: 'add_ai_summary_for_new_mr',

Currently the feature flag is enabled on production

It is possible that this MR will still need some changes to remove references to the feature flag in the code. At the moment the gitlab-housekeeper is not always capable of removing all references so you must check the diff and pipeline failures to confirm if there are any issues. It is the responsibility of groupcode creation to push those changes to this branch.

Note: If you do not want to remove this feature flag at this time, you can add an intended_to_rollout_by_date attribute in the feature flag YAML file to prevent automated removal.

TODO for the reviewers before merging this MR

• See the status of the rollout by checking gitlab-com/gl-infra/production#17533, https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues/?search=add_ai_summary_for_new_mr&sort=created_date&state=all&label_name%5B%5D=host%3A%3Agitlab.com
• Verify the feature flag status via chatops by running /chatops run feature get add_ai_summary_for_new_mr.
• Search for references to AddAiSummaryForNewMr in frontend part of code
• Search for references to add_ai_summary_for_new_mr in code
• Check if we need to remove any Gem or other related code by looking at the changes in !142739 (merged)

This change was generated by gitlab-housekeeper in CI using the Keeps::DeleteOldFeatureFlags keep.

To provide feedback on your experience with gitlab-housekeeper please create an issue with the label GitLab Housekeeper and consider pinging the author of this keep.