Don't export measurement azimuth help circle

This is a workaround for https://github.com/geoadmin/mf-geoadmin3/issues/1742

The trouble seems to be that features of type ol.geom.Circle can't be exported using ol3. At least, an exception is thrown when trying to export the cloned circle.

@oterral As this is hacky, could you shortly look for another solution? If not, please review/merge this.