collab: Remove GET /billing/monthly_spend endpoint (#31123)

This PR removes the `GET /billing/monthly_spend` endpoint, as it is no
longer used.

Release Notes:

- N/A

crates/collab/src/llm.rs

@@ -7,10 +7,6 @@ pub use token::*;
 
 pub const AGENT_EXTENDED_TRIAL_FEATURE_FLAG: &str = "agent-extended-trial";
 
-/// The maximum monthly spending an individual user can reach on the free tier
-/// before they have to pay.
-pub const FREE_TIER_MONTHLY_SPENDING_LIMIT: Cents = Cents::from_dollars(10);
-
 /// The default value to use for maximum spend per month if the user did not
 /// explicitly set a maximum spend.
 ///