Remove redundant lines in ReferenceExecutor

Somehow this was added in #1548 but I fail to see why.

Let's remove it.

This might be the cause for the performance degradation reported in #1548.

src/Executor/ReferenceExecutor.php

@@ -1350,9 +1350,6 @@ protected function executeFields(ObjectType $parentType, $rootValue, array $path
         $containsPromise = false;
         $results = [];
         foreach ($fields as $responseName => $fieldNodes) {
-            $fieldNodes = $fields[$responseName];
-            assert($fieldNodes instanceof \ArrayObject, 'The keys of $fields populate $responseName');
-
             $fieldPath = $path;
             $fieldPath[] = $responseName;