Remove an unused import (dart-lang/pub_semver#27)

diff --git a/pkgs/pub_semver/lib/src/utils.dart b/pkgs/pub_semver/lib/src/utils.dart
index adc1fae..55025a0 100644
--- a/pkgs/pub_semver/lib/src/utils.dart
+++ b/pkgs/pub_semver/lib/src/utils.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'package:collection/collection.dart';
-
 import 'version.dart';
 import 'version_range.dart';