collab: Remove lifetime spending limit in favor of LLM usage billing (#19321)

This PR removes the lifetime spending limit that was added in #16780.

We had previously added this as a way to prevent runaway usage, but now
that we have a cap on free usage per month with paid access after that,
we don't need this check anymore.

Release Notes:

- N/A

crates/collab/src/llm.rs

@@ -443,9 +443,6 @@ pub const FREE_TIER_MONTHLY_SPENDING_LIMIT: Cents = Cents::from_dollars(10);
 /// Used to prevent surprise bills.
 pub const DEFAULT_MAX_MONTHLY_SPEND: Cents = Cents::from_dollars(10);
 
-/// The maximum lifetime spending an individual user can reach before being cut off.
-const LIFETIME_SPENDING_LIMIT: Cents = Cents::from_dollars(1_000);
-
 async fn check_usage_limit(
     state: &Arc<LlmState>,
     provider: LanguageModelProvider,
@@ -487,14 +484,6 @@ async fn check_usage_limit(
         }
     }
 
-    // TODO: Remove this once we've rolled out monthly spending limits.
-    if usage.lifetime_spending >= LIFETIME_SPENDING_LIMIT {
-        return Err(Error::http(
-            StatusCode::FORBIDDEN,
-            "Maximum spending limit reached.".to_string(),
-        ));
-    }
-
     let active_users = state.get_active_user_count(provider, model_name).await?;
 
     let users_in_recent_minutes = active_users.users_in_recent_minutes.max(1);