common_audio: Removes unused macros

* WEBRTC_SPL_MUL_32_32_RSFT32BI
* WEBRTC_SPL_IS_NEG

BUG=3348
TESTED=trybots, common_audio_unittests
R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18449004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6169 4adac7df-926f-26a2-2b94-8c16560cd09d