Delete old placeholders for moved api/ header files

The header files

  api/congestion_control_interface.h
  api/data_channel_transport_interface.h
  api/datagram_transport_interface.h
  api/media_transport_config.h
  api/media_transport_interface.h

have been moved into the api/transport/ and api/transport/media
subdirectories.

Bug: webrtc:8733
Change-Id: I98752c4d1306b54559bafa71712b105932c08834
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/153522
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29357}
diff --git a/pc/sctp_utils.h b/pc/sctp_utils.h
index 6d41eb2..339ef21 100644
--- a/pc/sctp_utils.h
+++ b/pc/sctp_utils.h
@@ -14,7 +14,7 @@
 #include <string>
 
 #include "api/data_channel_interface.h"
-#include "api/data_channel_transport_interface.h"
+#include "api/transport/data_channel_transport_interface.h"
 #include "media/base/media_channel.h"
 
 namespace rtc {