Removal of more backend Visual Review code

What does this MR do and why?

This removes a bunch of dead code that's no longer used as of Remove visual review tools (!149586 - merged)

There was more removal than I thought, so we removed interfaces and anything user-facing in the first MR, and are sending follow-ups to do more removal behind the scenes.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Just run the test suite. All the changes in here are removals, so there's no additive thing to go look at. If everything else still works, we're good.

Related to #387751 (closed)

.rubocop_todo/rspec/before_all_role_assignment.yml

@@ -674,7 +674,6 @@ RSpec/BeforeAllRoleAssignment:
     - 'ee/spec/services/llm/merge_requests/summarize_review_service_spec.rb'
     - 'ee/spec/services/members/activate_service_spec.rb'
     - 'ee/spec/services/merge_trains/create_pipeline_service_spec.rb'
-    - 'ee/spec/services/notes/create_visual_review_service_spec.rb'
     - 'ee/spec/services/product_analytics/cube_data_query_service_spec.rb'
     - 'ee/spec/services/product_analytics/initialize_stack_service_spec.rb'
     - 'ee/spec/services/projects/deregister_suggested_reviewers_project_service_spec.rb'