Remove timestamp retreival warning/error.

An error reported while retreiving playout timestamp if no RTP packet received, yet. This causes an overflow of errors/warnings in applications where few channel are created but only one is actively engaged in a conversation. Therefore, we don't find such logging informative (there is no check upon correctness of timestamp computaion only if a packet already received).

BUG=3545
TEST=manual with voe_cmd_test,try bots
R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18869004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6781 4adac7df-926f-26a2-2b94-8c16560cd09d