Remove capture session js APIs

It's easy to implement it in pure js with the help of captureImage
method.  A js implementation makes it easy to syncronize camera
configuration and the capture.  With the old code you were never sure if
config changes were applied before the next shot.  Capture session APIs
were ugly because only one active session is supported (this restriction
is natural but the code enforcing the restriction is ugly; capturing
makes camera busy for extended periods of time hence concurrent capture
sessions would inevitably interfere).

There was a feature to trigger capture on motion detection.  This will
be separated into a new API allowing to establish a stream of
asynchronous motion detection notifications.