Delete unused rtpdump code in media/base.

Reading and writing RTP files is implemented elsewhere,
in test/rtp_file_reader.cc and test/rtp_file_writer.cc;
that code is untouched by this cl.

BUG=webrtc:6974

Review-Url: https://codereview.webrtc.org/2633453002
Cr-Original-Commit-Position: refs/heads/master@{#16046}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 61f31ee37607f5c3ba2c5930e6ab48f2b69b4c5f