collab: Remove Stripe code (#36275)

This PR removes the code for integrating with Stripe from Collab.

All of these concerns are now handled by Cloud.

Release Notes:

- N/A

crates/collab/src/api.rs

@@ -1,4 +1,3 @@
-pub mod billing;
 pub mod contributors;
 pub mod events;
 pub mod extensions;