Remove on_patch, on_put, ..., from NotificationsTypeV2API

The NotificationsType resource only implements on_get, https://github.com/openstack/monasca-api/blob/master/monasca_api/v2/reference/notificationstype.py. However, the interface at, https://github.com/openstack/monasca-api/blob/master/monasca_api/api/notificationstype_api_v2.py, implements a bunch of other methods such as on_put, on_post, ... Should remove all the extra ones as the spec only defined on_get for this resource.