Remove VertexAI prompts for chat

What does this MR do and why?

This MR removes all Vertex AI prompts for tools used in Chat.

This is because the chat's ai_request is now described as:

def ai_request
  ::Gitlab::Llm::Chain::Requests::AiGateway.new(user, tracking_context: tracking_context)
end

Source

and based on the code here

def provider_prompt_class
  ai_provider_name = ai_request.class.name.demodulize.underscore.to_sym

  self.class::PROVIDER_PROMPT_CLASSES[ai_provider_name]
end

the prompt provider class will always be obtained as self.class::PROVIDER_PROMPT_CLASSES[:ai_gateway], and hence Vertex AI prompts are redundant and can be removed.

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Related to #469377 (closed)