Delete context attrs instead of nullifying.

Author: umaannamalai

newrelic/api/llm_custom_attributes.py

@@ -41,4 +41,4 @@ def __enter__(self):
     def __exit__(self, exc, value, tb):
         # Clear out context attributes once we leave the current context
         if self.transaction:
-            self.transaction._llm_context_attrs = None
+            del self.transaction._llm_context_attrs