Make `destroy` redirect to agents list

Before this, the `destroy` action was attempting to redirect the user back to the `show` action of the controller, which resulted in an error.

Fixes #531
This change makes deleting an agent from within a Scenario redirect the
user to the Agents list. This might not be your intended behavior for this
action. I suggest making deleting an Agent from within a Scenario take you
back to the Scenario you deleted the Agent from within. Open to other ideas
though.