Delete the obsolete section, "track" and "stream" stats - they don't exist anymore

[henbos]

RTCMediaStreamTrackStats has been unshipped from Chromium already, and it is quite unhelpful to have an Obsolete section documenting metrics that don't exist anymore. Can we delete?

[henbos]

There's also "track" and "stream" being listed in RTCStatsType which is also just confusing

[henbos]

If we do this it should be quite easy to flatten the hierarchy if we so wish

[henbos]

The obsolete section made perfect sense when we were in the middle of changing everything around, but the spec has been quite stable for years now and the only piece that remained was deleting "track" and "stream", but with this having been removed already there is no value in this