* Remove parked layer feature.
Not worth the added complexity.
Several reasons
- Not seeing black frames on pub mute always.
- If they are there, it can consume more than 30kbps if the parked layer
is high res. That is wasted bandwidth downstream when pub is muted.
- On resume, client some time sends PLI and that triggers a key frame
request.
But, leaving the separate `PubMuted` flag in forwarder in case we can
use it for better handling.
* need the request spatial
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