[mediacapture-main] sourceType not used

stefhak has just created a new issue for 
https://github.com/w3c/mediacapture-main:

== sourceType not used ==
Among the current constrainable properties there is `sourceType`. 
However, it is not part of a `MediaStreamTrack`s 
constraints/supportedConstrainte/capabilities/settings and it is not 
usable with getUserMedia either.

The fact that no one has complained is to me an indication that we 
don't really need `sourceType`. I propose we simple remove it. Note 
that this would affect mediacapture-depth.

Please view or discuss this issue at 
https://github.com/w3c/mediacapture-main/issues/346 using your GitHub 
account