Remove unnecessary import (#175)

diff --git a/lib/src/empty_unmodifiable_set.dart b/lib/src/empty_unmodifiable_set.dart
index 44bd5ef..882b6c8 100644
--- a/lib/src/empty_unmodifiable_set.dart
+++ b/lib/src/empty_unmodifiable_set.dart
@@ -6,8 +6,6 @@
 
 import 'package:collection/collection.dart';
 
-import 'unmodifiable_wrappers.dart';
-
 /// An unmodifiable, empty set which can be constant.
 class EmptyUnmodifiableSet<E> extends IterableBase<E>
     with UnmodifiableSetMixin<E>