Pref service: delete old implementation

services/preferences/unittest_manifest.json

 {
   "name": "prefs_unittests",
   "display_name": "Prefs Unittests",
   "interface_provider_specs": {
     "service_manager:connector": {
       "provides": {
         "service_manager:service_factory": [
           "service_manager::mojom::ServiceFactory"
         ]
       },
       "requires": {
-        "preferences2": [ "pref_client", "pref_control" ]
+        "preferences": [ "pref_client", "pref_control" ]
       }
     }
   }
 }