Remove code to support browsers that don't have MessageChannel

Jasmine hasn't actually run in any such browsers since 2.x.

spec/core/StackClearerSpec.js

@@ -73,17 +73,6 @@ describe('StackClearer', function() {
         }
       };
     });
-
-    describe('when MessageChannel is unavailable', function() {
-      usesQueueMicrotaskWithSetTimeout(function() {
-        return {
-          navigator: {
-            userAgent: 'CERN-LineMode/2.15 libwww/2.17b3',
-            MessageChannel: undefined
-          }
-        };
-      });
-    });
   });
 
   describe('in Node', function() {