docs(contributing): remove reference to the obsolete chrome extension

Author: IgorMinar

CONTRIBUTING.md

@@ -143,14 +143,6 @@ from the main (upstream) repository:
     git pull --ff upstream master
     ```
 
-### GitHub Pull Request Helper
-
-We track Pull Requests by attaching labels and assigning to milestones.  For some reason GitHub
-does not provide a good UI for managing labels on Pull Requests (unlike Issues).  We have developed
-a simple Chrome Extension that enables you to view (and manage if you have permission) the labels
-on Pull Requests.  You can get the extension from the Chrome WebStore -
-[GitHub PR Helper][github-pr-helper]
-
 ## Coding Rules
 To ensure consistency throughout the source code, keep these rules in mind as you are working: