Clean up additional connection instrumentation attribute

The _djdt_chunked_cursor attribute wasn't being cleaned up in the
unwrap_cursor() method.

Author: living180

debug_toolbar/panels/sql/tracking.py

@@ -61,6 +61,7 @@ def chunked_cursor(*args, **kwargs):
 def unwrap_cursor(connection):
     if hasattr(connection, "_djdt_cursor"):
         del connection._djdt_cursor
+        del connection._djdt_chunked_cursor
         del connection.cursor
         del connection.chunked_cursor