collab: Remove unused RateLimiter (#29343)

This PR removes the `RateLimiter` from the collab codebase, as it is no
longer used.

Release Notes:

- N/A

crates/collab/src/main.rs

@@ -13,8 +13,8 @@ use collab::llm::db::LlmDatabase;
 use collab::migrations::run_database_migrations;
 use collab::user_backfiller::spawn_user_backfiller;
 use collab::{
-    AppState, Config, RateLimiter, Result, api::fetch_extensions_from_blob_store_periodically, db,
-    env, executor::Executor, rpc::ResultExt,
+    AppState, Config, Result, api::fetch_extensions_from_blob_store_periodically, db, env,
+    executor::Executor, rpc::ResultExt,
 };
 use collab::{ServiceMode, api::billing::poll_stripe_events_periodically};
 use db::Database;
@@ -111,10 +111,6 @@ async fn main() -> Result<()> {
 
                 if mode.is_collab() {
                     state.db.purge_old_embeddings().await.trace_err();
-                    RateLimiter::save_periodically(
-                        state.rate_limiter.clone(),
-                        state.executor.clone(),
-                    );
 
                     let epoch = state
                         .db