common_audio/signal_processing: Remove macro WEBRTC_SPL_SUB_SAT_W32

This macro is literally using the function WebRtcSpl_SubSatW32(), hence there is no need for a macro.

BUG=3348, 3353
TESTED=locally on Linux and trybots
R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/14149004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6899 4adac7df-926f-26a2-2b94-8c16560cd09d