Remove automatic session clearing on steps without a next property

[joefitter]

I'm not sure of the original intention of this but it seems to be causing more trouble than its worth.

If there is no next property the session data is cleared unless you pass clearSession: false in the step config. This is undocumented and I am unsure what the benefit of this is. IMO session clearing should be a deliberate action, not implicit if you forget to add clearSession: false