CONTRIBUTING: drop reference to CoreOS coding style

These docs are long gone.

Author: bgilbert

CONTRIBUTING.md

@@ -40,12 +40,6 @@ This is a rough outline of what a contributor's workflow looks like:
 
 Thanks for your contributions!
 
-### Coding Style
-
-CoreOS projects written in Go follow a set of style guidelines that we've documented 
-[here](https://github.com/coreos/docs/tree/master/golang). Please follow them when 
-working on your contributions.
-
 ### Format of the Commit Message
 
 We follow a rough convention for commit messages that is designed to answer two