Remove RCA experimental endpoint

Problem

In #441681 (closed) we developed a new Tool to power the RCA feature within duo chat. The frontend will still need to use the old api contract while the new feature is under development.

We will need to leave the beta endpoint in place to support the front end until the frontend is switched over.

Proposal

Delete the old beta code files:

• ee/lib/gitlab/llm/vertex_ai/completions/analyze_ci_job_failure.rb
• ee/app/services/llm/analyze_ci_job_failure_service.rb
• ee/app/graphql/types/ai/analyze_ci_job_failure_input_type.rb

and any other code related to this beta endpoint in these files:

• ee/app/helpers/ee/ci/jobs_helper.rb
• ee/app/services/llm/execute_method_service.rb
• ee/lib/gitlab/llm/completions_factory.rb
• ee/app/assets/javascripts/ci/job_details/components/sidebar/root_cause_analysis/graphql/root_cause.mutation.graphql