Remove END state from exploration graph

[GoogleCodeExporter]

At the moment, there are two ways to end an exploration: either through an 
'End' interaction, or by setting END as a destination. First, this is rather 
confusing, and second, the END state currently can't be edited. I would 
recommend removing the END state from the front-end altogether, so that the 
only way to end the exploration is through an 'End' interaction.

Original issue reported on code.google.com by amitdeut...@google.com on 8 Apr 2015 at 9:10