Clean directx_sdk_path as it is already defined in base/common.gypi

R=kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/23729004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7237 4adac7df-926f-26a2-2b94-8c16560cd09d