[pigeon] Removed gaaclarke from the readme (flutter#3395)

Author: gaaclarke

packages/pigeon/README.md

@@ -228,4 +228,4 @@ abstract class Api2Host {
 ## Feedback
 
 File an issue in [flutter/flutter](https://github.com/flutter/flutter) with the
-word 'pigeon' clearly in the title and cc **@gaaclarke**.
+word "pigeon" in the title.