webrtc::RtpPacket name freed for better RtpPacket

There were two different structures named RtpPacket in webrtc namespace:
RtpPacket defined in fec_test_helper renamed to test::RawRtpPacket
RtpPacket defined in rtp_sender_video and producer_fec removed as unused

BUG=webrtc:5261
R=sprang@google.com, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1710103004 .

Cr-Commit-Position: refs/heads/master@{#11682}

webrtc/modules/rtp_rtcp/source/producer_fec.h

@@ -18,8 +18,6 @@
 
 namespace webrtc {
 
-struct RtpPacket;
-
 class RedPacket {
  public:
   explicit RedPacket(size_t length);