Removing forwarding headers from api/

[Oskar Sundbom]

Hi,

In about two weeks, the following deprecated files will be removed from api/

• datachannel.h
  
• mediastream.h
  
• mediastreamtrack.h
  
• rtpsender.h
  
• streamcollection.h
  
• videotracksource.h
  
• webrtcsdp.h
  

The contents of these files have already moved, and these have been left in place only for backwards-compatibility. Since they are no longer part of our public API, they should not be used, especially not from their new locations.

Consider including the relevant interface headers instead, which are still part of our public API. In some cases, this may mean you'll have to create your own implementation of one or more of those interfaces. For tests, they could (and should?) be mocks.