Remove summarize_mr_changes in JH

this is follow up issue related to upstream issue.

remove feature in frontend: https://gitlab.com/gitlab-org/gitlab/-/issues/443331， mr: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/147116

remove feature in backend: https://gitlab.com/gitlab-org/gitlab/-/issues/451241

upstream remove frontend code, JH waiting upstream remove backend code, then JH can start modifying code according to it.