camerabin2: Update v4l2camerasrc to use start/stop signals

camerabin2: Update v4l2camerasrc to use start/stop signals

Removes the old logic for v4l2camerasrc that used the mode
property switching to start/stop captures to make it identical
to camerabin2 behavior and to allow the future addition of
pausing a video recording.

This also removes the MODE_PREVIEW as it became useless.