rpc: Remove llm module in favor of zed_llm_client (#28900)

This PR removes the `llm` module of the `rpc` crate in favor of using
the types from the `zed_llm_client`.

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -330,8 +330,10 @@ async fn create_billing_subscription(
                 .await?
         }
         None => {
-            let default_model =
-                llm_db.model(rpc::LanguageModelProvider::Anthropic, "claude-3-7-sonnet")?;
+            let default_model = llm_db.model(
+                zed_llm_client::LanguageModelProvider::Anthropic,
+                "claude-3-7-sonnet",
+            )?;
             let stripe_model = stripe_billing.register_model(default_model).await?;
             stripe_billing
                 .checkout(customer_id, &user.github_login, &stripe_model, &success_url)