Prompt Migration: Remove VertexAI prompts for chat

Overview

We have VertexAI prompts in GitLab Rails codebase that are not used anymore and can be removed. Examples:

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/gitlab/llm/chain/tools/ci_editor_assistant/prompts/vertex_ai.rb
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/gitlab/llm/chain/tools/epic_reader/prompts/vertex_ai.rb
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/gitlab/llm/chain/tools/issue_reader/prompts/vertex_ai.rb
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/gitlab/llm/chain/tools/refactor_code/prompts/vertex_ai.rb
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/gitlab/llm/chain/tools/write_tests/prompts/vertex_ai.rb
• https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/gitlab/llm/chain/tools/explain_code/prompts/vertex_ai.rb

Proposal

Let's clean them up as a prerequisite to Prompt Migration to AI Gateway (&14259 - closed)