Add destroy note subscription api endpoint

app/controllers/api/note_subscriptions_controller.rb

@@ -14,5 +14,14 @@ module Api
     rescue ActiveRecord::RecordNotUnique
       report_error "You are already subscribed to note #{note_id}.", :conflict
     end
+
+    def destroy
+      note_id = params[:note_id].to_i
+      note = Note.find(note_id)
+      count = note.subscriptions.where(:user => current_user).delete_all
+      report_error "You are not subscribed to note #{note_id}.", :not_found if count.zero?
+    rescue ActiveRecord::RecordNotFound
+      report_error "Note #{note_id} not found.", :not_found
+    end
   end
 end