feat(DuoChat): Remove "generated by AI" message

mentioned in commit 5a4af915

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1011 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!166770 (closed)

mentioned in merge request gitlab!166793 (merged)

mentioned in merge request gitlab!166867 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!349 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!51 (merged)