collab: Remove GET /billing/preferences endpoint (#34566)

This PR removes the `GET /billing/preferences` endpoint, as it has been
moved to `cloud.zed.dev`.

Release Notes:

- N/A

crates/collab/src/lib.rs

@@ -1,6 +1,5 @@
 pub mod api;
 pub mod auth;
-mod cents;
 pub mod db;
 pub mod env;
 pub mod executor;
@@ -21,7 +20,6 @@ use axum::{
     http::{HeaderMap, StatusCode},
     response::IntoResponse,
 };
-pub use cents::*;
 use db::{ChannelId, Database};
 use executor::Executor;
 use llm::db::LlmDatabase;