[CLEANUP] Remove duplicate default export in snapshot.js

`Snapshot` is default export-ed already at the end of the file.

packages/ember-data/lib/system/snapshot.js

@@ -12,7 +12,7 @@ var get = Ember.get;
   @constructor
   @param {DS.Model} internalModel The model to create a snapshot from
 */
-export default function Snapshot(internalModel) {
+function Snapshot(internalModel) {
   this._attributes = new EmptyObject();
   this._belongsToRelationships = new EmptyObject();
   this._belongsToIds = new EmptyObject();