removed GAIN preprocessor definition, removed needsgain from struct wav_desc,...

removed GAIN preprocessor definition, removed needsgain from struct wav_desc, removed unnecessary gain code from wav_read() and wav_write()

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@60641 65c4cc65-6c06-0410-ace0-fbb531ad65f3