Now that Chromium has taken libsrtp2, remove any compatibility bridge code in WebRTC that was only needed for libsrtp1.
Remove SRTP_RELATIVE_PATH now that Google's internal copy of libsrtp and the Chromium copy have the same directory structure.
Fix some include orderings per the Chromium C++ style guide.
Remove the `extern "C"` blocks now that the libsrtp headers include them (https://github.com/cisco/libsrtp/pull/195).
BUG=webrtc:6376
Review-Url: https://codereview.webrtc.org/2447893002
Cr-Commit-Position: refs/heads/master@{#14776}
0d8ade543d