collab: Unconditionally execute billing checks (#19432)

This PR removes the conditional checks around the billing-related
enforcement for LLM completions.

These were just in place to prevent executing any billing code before we
had rolled it out. Now that it is rolled out, we don't need this
conditional execution anymore.

Release Notes:

- N/A

crates/collab/src/lib.rs

@@ -198,10 +198,6 @@ impl Config {
         }
     }
 
-    pub fn is_llm_billing_enabled(&self) -> bool {
-        self.stripe_api_key.is_some()
-    }
-
     #[cfg(test)]
     pub fn test() -> Self {
         Self {