Remove unnecessary RTPHeaderParser, following https://codereview.webrtc.org/2659563002/

BUG=webrtc:6847

Review-Url: https://codereview.webrtc.org/2663063008
Cr-Original-Commit-Position: refs/heads/master@{#16457}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6b34124a6d4733cf2d95986b4920bccec8b39397
3 files changed