Change where we listen to `relational:unregister`.
Change where we listen to `relational:unregister`.

So it doesn't depend on a model being registered (when it doesn't have an id yet), and we'll still remove existing event listeners. Ref #423.

airbnb/Backbone-relational,BothMedia/Backbone-relational,PaulUithol/Backbone-relational,matghaleb/Backbone-relational,matghaleb/Backbone-relational,nikolas/Backbone-relational,goodwall/Backbone-relational,Feasul/Backbone-relational,PaulUithol/Backbone-relational,nikolas/Backbone-relational,xcambar/Backbone-relational,leeleo26/Backbone-relational,Feasul/Backbone-relational,goodwall/Backbone-relational,closeio/Backbone-relational,leeleo26/Backbone-relational,closeio/Backbone-relational,xcambar/Backbone-relational