[mediacapture-main] Proposal: Remove onoverconstrained

getUserMedia() gives you a track with the capabilities/settings you asked
for. Even with the right settings, you might not get what you asked for.
Example: poor lighting condition => less FPS than the camera aims for. This
triggers “onoverconstrained”.

Problems:

   -

   Overconstrained mutes the track… (foot-gun!)
   -

      …making it unusable (silent/black).
      -

      …which contradicts the definition of “mute”, which is:
      The muted/unmuted state of a track reflects whether the source
      provides any media at this moment.
      -

      (Separate discussion: If applicable to remote WebRTC tracks, “onmute”
      has a different meaning: negotiation triggered it not being used to
      receive.)
      -

   Quoting Jan-Ivar
   <https://github.com/w3c/mediacapture-main/issues/573#issuecomment-470700204>
   :
   -

      It's redundant: just measure the output directly and react to it.
      -

      It's undesired: demand has not materialized in 5 years.


*Proposal:*

Remove "onoverconstrained" from the spec.


No browser has implemented it and there seem to already be consensus
<https://github.com/w3c/mediacapture-main/issues/573> about removing it
among browser implementors and editors. If you don't agree, speak up.


(Slides 38-39 of WebRTC March Virtual Interim
<https://docs.google.com/presentation/d/1RmKf-Fb10pF0_P8YwMVCnH2lV7_2_dDREiGI1tY1zl0/edit?usp=sharing>
covers
this topic, but we never had the time to discuss it then.)