Remove link to IRC in CONTRIBUTING.md, not used anymore

Author: brentvatne

CONTRIBUTING.md

@@ -46,7 +46,6 @@ Facebook has a [bounty program](https://www.facebook.com/whitehat/) for the safe
 
 ## How to Get in Touch
 
-* IRC - [#reactnative on freenode](http://webchat.freenode.net/?channels=reactnative)
 * [Facebook group](https://www.facebook.com/groups/react.native.community/)
 * Reactiflux — [#react-native](http://join.reactiflux.com/)