Don't drop editor subscriptions in its destroyed hook

Unsubscribing should be the responsibility of the subscribers and doing
this was removing 'destroyed' subscriptions before the 'destroyed' event
was emitted now that 'destroyed' is emitted by the default
Model::destroy method.

src/editor.coffee

@@ -124,7 +124,6 @@ class Editor extends Model
     @displayBuffer.destroy()
     @languageMode.destroy()
     atom.project?.removeEditor(this)
-    @off()
 
   # Private: Creates an {Editor} with the same initial state
   copy: ->