Fix linter issue.

Author: WaVEV

django_mongodb/query_utils.py

@@ -1,6 +1,6 @@
 from django.core.exceptions import FullResultSet
 from django.db.models.aggregates import Aggregate
-from django.db.models.expressions import Expression, Value
+from django.db.models.expressions import Value
 
 
 def is_direct_value(node):