polish(incremental): remove unnecessary helper

Replicates graphql/graphql-js@d57d5c7

Author: Cito

src/graphql/execution/incremental_graph.py

@@ -135,7 +135,7 @@ def complete_deferred_fragment(
         successful_execution_groups = list(
             deferred_fragment_record.successful_execution_groups
         )
-        self._remove_root_node(deferred_fragment_record)
+        del self._root_nodes[deferred_fragment_record]
         for successful_execution_group in successful_execution_groups:
             pending_execution_group = successful_execution_group.pending_execution_group
             deferred_records = pending_execution_group.deferred_fragment_records
@@ -156,22 +156,18 @@ def remove_deferred_fragment(
         """Check if deferred fragment exists and remove it in that case."""
         if deferred_fragment_record not in self._root_nodes:
             return False
-        self._remove_root_node(deferred_fragment_record)
+        del self._root_nodes[deferred_fragment_record]
         return True
 
     def remove_stream(self, stream_record: StreamRecord) -> None:
         """Remove a stream record as no longer pending."""
-        self._remove_root_node(stream_record)
+        del self._root_nodes[stream_record]
 
     def stop_incremental_data(self) -> None:
         """Stop the delivery of incremental data."""
         for future in self._next_queue:
             future.cancel()  # pragma: no cover
 
-    def _remove_root_node(self, delivery_group: DeliveryGroup) -> None:
-        """Remove root node."""
-        del self._root_nodes[delivery_group]
-
     def _add_incremental_data_records(
         self,
         incremental_data_records: Sequence[IncrementalDataRecord],