docs(contributing): remove link to google group

fix angular-ui#1391

Author: ProLoser

CONTRIBUTING.md

@@ -13,8 +13,7 @@ and provides a bit of background as to the circumstances that triggered it. Don'
 that plunkr you created!
 
 **Note**: If you're unsure how a feature is used, or are encountering some unexpected behavior that you aren't sure
-is a bug, it's best to talk it out in the
-[Google Group](https://groups.google.com/forum/#!categories/angular-ui/router) or on
+is a bug, it's best to talk it out on
 [StackOverflow](http://stackoverflow.com/questions/ask?tags=angularjs,angular-ui-router) before reporting it. This
 keeps development streamlined, and helps us focus on building great software.