Follow-up prompt migration: remove GitLab Rails Anthropic prompt templates

Problem to solve

Blueprint: https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/prompts_migration/

Post-migration of each tool's prompt to the AI Gateway, we now have dead code within each GitLab anthropic template. We can remove the following dead code to make it more explicit that we no longer doing variable expansion with the HERDOC templates.

Example Issue Reader Prompt Template

 ```
   PROMPT_TEMPLATE = [
          Utils::Prompt.as_system(
            <<~PROMPT
              You are a software developer.
              You can explain code snippets.
              %<language_info>s
            PROMPT
          ),
          Utils::Prompt.as_user(
            <<~PROMPT.chomp
              %<file_content>s
              Here is the code user selected:
              <selected_code>
                %<selected_text>s
              </selected_code>

              %<input>s
              Any code blocks in response should be formatted in markdown.
            PROMPT
          )
        ].freeze
```

Proposal

For all the ee/lib/gitlab/llm/chain/<tool>/prompts, remove the PROMPT_TEMPLATE or any HEREDOC that is no longer in use.

Definition of Done

• Tool calling to the AI Gateway still work with just passing in the options[:input] to the AI-Gateway.
• Make sure that we aren't causing any regressions with current tooling.