collab: Set plan in LLM token based on subscription (#29231)

This PR updates the `plan` field in the LLM token to be based on the
subscription.

We weren't using this field anywhere outside of the new billing code, so
it is safe to change its meaning.

Release Notes:

- N/A

crates/collab/src/rpc.rs

@@ -4147,7 +4147,6 @@ async fn get_llm_api_token(
         billing_preferences,
         &flags,
         has_legacy_llm_subscription,
-        session.current_plan(&db).await?,
         billing_subscription,
         session.system_id.clone(),
         &session.app_state.config,