Removing neteq decode lock and friends

NetEq is thread-safe by virtue of it's own lock, and in r6404 the
ACMISAC class was made thread-safe. Therefore, the neteq decode lock
is no longer needed.

R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18599004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6531 4adac7df-926f-26a2-2b94-8c16560cd09d