collab: Remove unused billing-related database code (#36282)

This PR removes a bunch of unused database code related to billing, as
we no longer need it.

Release Notes:

- N/A

crates/collab/src/tests/test_server.rs

@@ -565,7 +565,6 @@ impl TestServer {
     ) -> Arc<AppState> {
         Arc::new(AppState {
             db: test_db.db().clone(),
-            llm_db: None,
             livekit_client: Some(Arc::new(livekit_test_server.create_api_client())),
             blob_store_client: None,
             executor,