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/llm.rs

@@ -1,8 +1,6 @@
 pub mod db;
 mod token;
 
-use crate::Cents;
-
 pub use token::*;
 
 pub const AGENT_EXTENDED_TRIAL_FEATURE_FLAG: &str = "agent-extended-trial";
@@ -12,9 +10,3 @@ pub const BYPASS_ACCOUNT_AGE_CHECK_FEATURE_FLAG: &str = "bypass-account-age-chec
 
 /// The minimum account age an account must have in order to use the LLM service.
 pub const MIN_ACCOUNT_AGE_FOR_LLM_USE: chrono::Duration = chrono::Duration::days(30);
-
-/// The default value to use for maximum spend per month if the user did not
-/// explicitly set a maximum spend.
-///
-/// Used to prevent surprise bills.
-pub const DEFAULT_MAX_MONTHLY_SPEND: Cents = Cents::from_dollars(10);