The mic element sat in the top-right corner of every stream that opened a
capture — a standing button on touch, a Muted badge on TV. It goes for now;
the on-screen overlay UI being built will carry mute as one of its controls,
and re-introducing it there is the right moment to decide how it looks.
Mute itself is untouched: `micRunning`, `micMuted` and `setMicMuted` still
back the Select + Y chord, which is now the whole of the control, and
`MicChordHint` is now its only on-screen feedback (its doc updated to say so
rather than pointing at the badge that no longer exists).
enricobuehler