Delete unused method PayloadRouter::MaxPayloadLength.

Documentation was also unclear, it seems it returned the RTP packet
size including RTP headers.

BUG=None.

Review-Url: https://codereview.webrtc.org/2588343002
Cr-Original-Commit-Position: refs/heads/master@{#15707}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5206667dadd69d30309dc69cc866925299b0efe1
3 files changed