Remove DestructEncoderInst and its codec-specific implementations.

This method is seemingly never called.

BUG=none
TEST=none
R=henrik.lundin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/24539004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7131 4adac7df-926f-26a2-2b94-8c16560cd09d