Cleaned up real_fft APIs due to non-existing NEON code

There are NEON APIs that are not used. Cleaning that up for better overview.

BUG=3353
TESTED=locally on Linux and trybots
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/31149004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7827 4adac7df-926f-26a2-2b94-8c16560cd09d