Remove documentation for Codebase Search MCP tool

What does this MR do and why?

A documentation for the Codebase Search MCP tool was introduced in Add `get_code_context` tool to MCP sever docs (!207617 - merged).

However, this tool is not ready.

• This is behind a disabled Feature Flag
• This is backed by Code Embeddings Index, which we haven't announced as available yet, even internally
• This tool needs some further tweaks, which we are experimenting with on [Agentic Chat Integration] Test Codebase Search... (#572264 - closed)
  • additionally: we might still need to rename this tool

We should remove the documentation for this tool until we can deem that it's ready to use. This includes by internal GitLab users. Currently, we haven't made this tool available even for internal GitLab users.

References

See my comment in !207617 (comment 2802963982)

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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