Remove spurious storage.fetch from migration

This should really only be called once, from background.js.

Calling it twice can cause doubled listeners for the registration_done
event, which in turn leads to duplicate post-registration callbacks,
dual sync requests, and an eventual datastore inconsistency.

Fixes #670

// FREEBIE

js/database.js

@@ -107,7 +107,6 @@
                         next();
                     }
                 });
-                storage.fetch();
                 next();
             }
         },