Remove engine_codecs_ cache from unittests.

Used interchangably with engine_.codecs() becomes confusing and it's not
really used that much.

BUG=1788
R=pthatcher@google.com, pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17689005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6429 4adac7df-926f-26a2-2b94-8c16560cd09d