Disable legacy global microphone/webcam indicator

This indicator appears when webcame or microphone is being used by Mullvad Browser:

If we wanted we could probably update the background colour/icons if we want to make them more mullvad (eg purple or blue instead of firefox yellow/orange).

Does this seem like something y'all would want to update @ruihildt

/cc @donuts @

EDIT: It's been decided to disable this feature. Per the linked stackoverflow question we should be able to just set privacy.webrtc.legacyGlobalIndicator to false.