Remove unused RtpStatistics struct.

This unused struct is basically a copy of RtcpStatistics in
webrtc/common_types.h. I expect this wasn't properly removed when that
one was added.

R=tommi@webrtc.org
BUG=

Review URL: https://webrtc-codereview.appspot.com/25239004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7758 4adac7df-926f-26a2-2b94-8c16560cd09d