Tracing: Remove backend application code

Why are we doing this work

In 15.0, we disabled the Tracing features via a feature flag. After the feature flag is disabled & frontend code is deleted, the backend pieces can be cleaned up.

⚠ Removal should take place in %15.2 or later.

Tracking issue for deprecation & removal: Monitor: Tracing Deprecation (#346540 - closed)

Implementation plan

1. Ensure all tracing-related code is removed
2. Database cleanup will occur as part of a separate issue

Relevant links

• app/controllers/projects/settings/operations_controller.rb - !90567 (merged)
• app/controllers/projects/tracings_controller.rb - !90628 (merged)
• app/helpers/projects_helper.rb - !90628 (merged)
• app/models/project_tracing_setting.rb - !90628 (merged)
• app/models/project.rb - !90628 (merged)
• app/services/projects/operations/update_service.rb - !90628 (merged)
• config/routes/project.rb - !90662 (merged)
• ee/app/controllers/ee/projects/settings/operations_controller.rb - !90567 (merged)
• spec/controllers/projects/settings/operations_controller_spec.rb - !90567 (merged)
• spec/controllers/projects/tracings_controller_spec.rb - !90628 (merged)
• spec/factories/project_tracing_settings.rb - !90662 (merged)
• spec/factories/usage_data.rb - !90662 (merged)
• spec/features/projects/tracings_spec.rb - !90628 (merged)
• spec/lib/gitlab/usage_data_spec.rb - !90607 (merged)
• spec/models/project_spec.rb - !90628 (merged)
• spec/models/project_tracing_setting_spec.rb - !90628 (merged)
• spec/services/projects/operations/update_service_spec.rb - !90628 (merged)
• spec/support/helpers/usage_data_helpers.rb - !90662 (merged)
• .rubocop_todo/gitlab/namespaced_class.yml - !90628 (merged)
• lib/gitlab/database/gitlab_schemas.yml - !90628 (merged)
• spec/lib/gitlab/import_export/all_models.yml - !90628 (merged)
• spec/lib/gitlab/import_export/safe_model_attributes.yml - !90628 (merged)