Remove duplicate skymaps in circulars

• Fix duplicate skymap references in the automated circulars. This happened in the past because of bad connections with GCN where multiple VOEvents were produced in succession with the same skymap in hand....

Then we got things like:

Nine sky maps are available at this time and can be retrieved from the GraceDB event page:

• bayestar.fits.gz, an updated localization generated by BAYESTAR, distributed via GCN notice about 4 hours after the event
• bayestar.fits.gz, an initial localization generated by BAYESTAR, distributed via GCN notice about 5 hours after the event
• bayestar.fits.gz, an initial localization generated by BAYESTAR, distributed via GCN notice about 6 hours after the event
• bayestar.fits.gz, an updated localization generated by BAYESTAR, distributed via GCN notice about 2 days after the event
• LALInference_skymap.fits.gz, an updated localization generated by LIB, distributed via GCN notice about 2 days after the event
• bayestar.fits.gz, an updated localization generated by BAYESTAR, distributed via GCN notice about 2 days after the event
• LALInference_r2.fits.gz, an updated localization generated by UNKNOWN, distributed via GCN notice about 2 days after the event
• LALInference_r2.fits.gz, an updated localization generated by UNKNOWN, distributed via GCN notice about 15 days after the event
• LALInference_f.fits.gz, an updated localization generated by LIB, distributed via GCN notice about 3 months after the event

Where a lot of these are duplicates and need to be removed!!