Fix release method of incremental publisher

Author: Cito

src/graphql/execution/incremental_publisher.py

@@ -511,7 +511,6 @@ def _introduce(self, item: IncrementalDataRecord) -> None:
 
     def _release(self, item: IncrementalDataRecord) -> None:
         if item in self._pending:
-            del self._pending[item]
             self._released[item] = None
             self._trigger()