agent: Remove unused max monthly spend reached error (#30615)

This PR removes the code for showing the max monthly spend limit reached
error, as it is no longer used.

Release Notes:

- N/A

crates/language_model/src/model/cloud_model.rs

@@ -133,18 +133,6 @@ impl fmt::Display for PaymentRequiredError {
     }
 }
 
-#[derive(Error, Debug)]
-pub struct MaxMonthlySpendReachedError;
-
-impl fmt::Display for MaxMonthlySpendReachedError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(
-            f,
-            "Maximum spending limit reached for this month. For more usage, increase your spending limit."
-        )
-    }
-}
-
 #[derive(Error, Debug)]
 pub struct ModelRequestLimitReachedError {
     pub plan: Plan,