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.cc

@@ -32,11 +32,6 @@ enum { kHighProtectionThreshold = 80 };  // Corresponds to ~30 overhead, range
 // is 0 to 255, where 255 corresponds to 100% overhead (relative to number of
 // media packets).
 
-struct RtpPacket {
-  uint16_t rtpHeaderLength;
-  ForwardErrorCorrection::Packet* pkt;
-};
-
 RedPacket::RedPacket(size_t length)
     : data_(new uint8_t[length]),
       length_(length),