Removing macro in acm_opus.cc

Remove it since macros are not recommended to use according to code style guide.

BUG=
R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/21219004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6917 4adac7df-926f-26a2-2b94-8c16560cd09d