Update README.md

Removed OpenBSD line per discussion started with https://github.com/ipfs/docs/issues/254#issuecomment-526372366

docs/README.md

@@ -26,7 +26,6 @@ Otherwise, check out the following guides to using and developing IPFS:
 
 - Building on…
     - [Windows](windows.md)
-    - [OpenBSD](openbsd.md)
 - [Performance Debugging Guidelines](debug-guide.md)
 - [Release Checklist](releases.md)