[MediaStream] remove MediaStream.label
https://bugs.webkit.org/show_bug.cgi?id=121337

Reviewed by Sam Weinig.

No new tests or modified tests, this attribute was apparently never tested.

* Modules/mediastream/MediaStream.h:
* Modules/mediastream/MediaStream.idl:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@155789 268f45cc-cd09-0410-ab3c-d52691b4dbfc