Remove string index from fragment registry (adopted-ember-addons#455)

Author: charlesfries

types/registries/fragment.d.ts

@@ -1,8 +1,4 @@
-import Fragment from 'ember-data-model-fragments/fragment';
-
 /**
  * Define all the models here.
  */
-export default interface FragmentRegistry {
-  [key: string]: Fragment;
-}
+export default interface FragmentRegistry {}