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

crates/proto/src/proto.rs

@@ -50,8 +50,6 @@ messages!(
     (CloseBuffer, Foreground),
     (Commit, Background),
     (CopyProjectEntry, Foreground),
-    (CountLanguageModelTokens, Background),
-    (CountLanguageModelTokensResponse, Background),
     (CreateBufferForPeer, Foreground),
     (CreateChannel, Foreground),
     (CreateChannelResponse, Foreground),
@@ -374,7 +372,6 @@ request_messages!(
     (PerformRename, PerformRenameResponse),
     (Ping, Ack),
     (PrepareRename, PrepareRenameResponse),
-    (CountLanguageModelTokens, CountLanguageModelTokensResponse),
     (RefreshInlayHints, Ack),
     (RefreshCodeLens, Ack),
     (RejoinChannelBuffers, RejoinChannelBuffersResponse),