Remove reference to ansible-validate-modules (#3256)

ansible-validate-modules is now in ansible/ansible

During 2.3 we will be merge the modules into ansible/ansible so this file will go away.

The new testing documentation will refer to `ansible-test` which will wrap up the unit, integration, and ansible-validate-modules. So no need to document here.

lib/ansible/modules/extras/GUIDELINES.md

@@ -64,7 +64,6 @@ If you'd like to step down as a maintainer, please submit a PR to the maintainer
 
 ## Useful tools
 * https://ansible.sivel.net/pr/byfile.html -- a full list of all open Pull Requests, organized by file. 
-* https://github.com/sivel/ansible-testing -- these are the tests that run on Shippable against all PRs for extras modules, so it's a good idea to run these tests locally first.
 
 ## Other Resources