[Form] Removed undefined variables in exception constructor

src/Symfony/Component/Form/Extension/Core/DataTransformer/ChoicesToBooleanArrayTransformer.php

@@ -107,7 +107,7 @@ class ChoicesToBooleanArrayTransformer implements DataTransformerInterface
         }
 
         if (count($unknown) > 0) {
-            throw new TransformationFailedException('The choices "' . implode('", "', $unknown, $code, $previous) . '" where not found');
+            throw new TransformationFailedException('The choices "' . implode('", "', $unknown) . '" where not found');
         }
 
         return $result;