collab: Remove CountLanguageModelTokens RPC message (#29314)

This PR removes the `CountLanguageModelTokens` RPC message from collab.

We were only using this for Google AI models through the Zed provider
(which is only available to Zed staff).

For now we're returning `0`, but will bring back soon.

Release Notes:

- N/A

Cargo.lock

@@ -2998,7 +2998,6 @@ dependencies = [
  "git",
  "git_hosting_providers",
  "git_ui",
- "google_ai",
  "gpui",
  "gpui_tokio",
  "hex",