remove placeholder for non-linux platforms

runc currently only support Linux platform, and since we dont intend to expose
the support to other platform, removing all other platforms placeholder code.

`libcontainer/configs` still being used in
https://github.com/moby/moby/blob/master/daemon/daemon_windows.go so
keeping it for now.

After this, we probably should also rename files to drop linux suffices
if possible.

Signed-off-by: Daniel Dao <[email protected]>

Author: dqminh

libcontainer/configs/cgroup_unsupported.go

@@ -1,5 +1,3 @@
-// +build linux
-
 package devices
 
 import (