Remove @Nullable annotations on region parameters

Regions should never be null in OpenStack and all of the annotations have been removed. If someone happens to pass null, an exception will be thrown in [RegionToEndpoint](https://github.com/jclouds/jclouds/blob/4c74b497547e42b8bdc94dbae3d4cd94ff3945d6/core/src/main/java/org/jclouds/location/functions/RegionToEndpoint.java#L47).