Removing broken GAMMA link

Author: robscott

mkdocs.yml

@@ -123,7 +123,6 @@ nav:
     - How to Get Involved: contributing/index.md
     - Developer Guide: contributing/devguide.md
     - Enhancement Requests: contributing/enhancement-requests.md
-    - GAMMA: contributing/gamma.md
     - Contributor Ladder: contributing/contributor-ladder.md
   - Blog:
     - Index: blog/index.md