fix(webcams): avoid leaving dangling HTMLMediaElements in paused state

Should fix an issue with the recent Chrome 92 intervention that limits
the number of concurrent WebMediaPlayers (an inner element of
HTMLMediaElements) to 75/40.

Webcam video elements were being left dangling in paused state despite
the elements themselves being cleaned up from the component. That
generated a skewed accounting of WebMediaPlayers in the session.