removing service method check so we can add hooks to custom methods

I'm not where we can really do the check to see if the original service method exists since we don't have context for `this._super`. Possibly we could add the check [here](https://github.com/feathersjs/feathers-hooks/blob/master/lib/after.js#L74) but then the hook will still get added. It will just never call the super method.
:thumbsup: