Remove VertexAI prompts for chat

ee/lib/gitlab/llm/chain/concerns/vertex_ai_prompt.rb

...ib/gitlab/llm/chain/tools/ci_editor_assistant/executor.rb

.../llm/chain/tools/ci_editor_assistant/prompts/vertex_ai.rb

ee/lib/gitlab/llm/chain/tools/epic_reader/executor.rb

...b/gitlab/llm/chain/tools/epic_reader/prompts/vertex_ai.rb

ee/lib/gitlab/llm/chain/tools/explain_code/executor.rb

.../gitlab/llm/chain/tools/explain_code/prompts/vertex_ai.rb

ee/lib/gitlab/llm/chain/tools/fix_code/executor.rb

ee/lib/gitlab/llm/chain/tools/fix_code/prompts/vertex_ai.rb

ee/lib/gitlab/llm/chain/tools/issue_reader/executor.rb

.../gitlab/llm/chain/tools/issue_reader/prompts/vertex_ai.rb

ee/lib/gitlab/llm/chain/tools/refactor_code/executor.rb

...gitlab/llm/chain/tools/refactor_code/prompts/vertex_ai.rb

ee/lib/gitlab/llm/chain/tools/write_tests/executor.rb

...b/gitlab/llm/chain/tools/write_tests/prompts/vertex_ai.rb

...chain/tools/ci_editor_assistant/prompts/vertex_ai_spec.rb

...lab/llm/chain/tools/epic_reader/prompts/vertex_ai_spec.rb

...ab/llm/chain/tools/explain_code/prompts/vertex_ai_spec.rb

...gitlab/llm/chain/tools/fix_code/prompts/vertex_ai_spec.rb

...ab/llm/chain/tools/issue_reader/prompts/vertex_ai_spec.rb