Remove support for the DeferredLibrary annotation and remove tests of it

tests/language/deferred_constraints_old_syntax_lib.dart

Index: tests/language/deferred_constraints_old_syntax_lib.dart
diff --git a/tests/language/deferred_constraints_old_syntax_lib.dart b/tests/language/deferred_constraints_old_syntax_lib.dart
deleted file mode 100644
index 0bf3cc9724574d4e39185ece3c8358d261123454..0000000000000000000000000000000000000000
--- a/tests/language/deferred_constraints_old_syntax_lib.dart
+++ /dev/null
@@ -1,17 +0,0 @@
-// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
-// 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.
-
-class C {
-  static int staticMethod() => 42;
-}
-
-class G<T> {}
-
-class Const {
-  const Const();
-  const Const.otherConstructor();
-  static const instance = const Const();
-}
-
-const constantInstance = const Const();