Remove `skymap_type` from `createVOEvent`
python3 branch for GraceDB server includes a few minor changes to the VOEvent file format, including the fact that there won't be any need for the
skymap_type any longer. We should deprecate it in one release (i.e., make it non-required and print a warning if it's included in the kwargs to
createVOEvent), and then remove it in the next release.