This ensures that the figure in the notebook is closed correctly when closed

I.e.

plt.plot(range(10))

creates a figure and then in a new cell assuming this is the first figure we can close it with:

plt.close(1)

Currently this does not have any visual effect. This change ensures that the figure is closed in the notebook as expected.

I have removed the self._send_event('close') because it seems to trigger a no comms warning. As I understand it this no longer needed anyway as the widget handles the closing.