Remove skymap_png parameter from VOEvents

The sky map plots take longer to generate than the FITS files
themselves, so they would have needlessly delayed the preliminary
alerts.

_static/MS181101ab-1-Preliminary.xml

@@ -54,9 +54,6 @@ xsi:schemaLocation="http://www.ivoa.net/xml/VOEvent/v2.0 http://www.ivoa.net/xml
             <Param name="skymap_fits" dataType="string" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.fits.gz" ucd="meta.ref.url">
                 <Description>Sky Map FITS</Description>
             </Param>
-            <Param name="skymap_png" dataType="string" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.png" ucd="meta.ref.url">
-                <Description>Sky Map image</Description>
-            </Param>
         </Group>
         <Group type="Classification">
             <Description>

_static/MS181101ab-2-Initial.xml

@@ -54,9 +54,6 @@ xsi:schemaLocation="http://www.ivoa.net/xml/VOEvent/v2.0 http://www.ivoa.net/xml
             <Param name="skymap_fits" dataType="string" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.fits.gz" ucd="meta.ref.url">
                 <Description>Sky Map FITS</Description>
             </Param>
-            <Param name="skymap_png" dataType="string" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.png" ucd="meta.ref.url">
-                <Description>Sky Map image</Description>
-            </Param>
         </Group>
         <Group type="Classification">
             <Description>

_static/MS181101ab-3-Update.xml

@@ -54,9 +54,6 @@ xsi:schemaLocation="http://www.ivoa.net/xml/VOEvent/v2.0 http://www.ivoa.net/xml
             <Param name="skymap_fits" dataType="string" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.fits.gz" ucd="meta.ref.url">
                 <Description>Sky Map FITS</Description>
             </Param>
-            <Param name="skymap_png" dataType="string" value="https://emfollow.docs.ligo.org/userguide/_static/bayestar.png" ucd="meta.ref.url">
-                <Description>Sky Map image</Description>
-            </Param>
         </Group>
         <Group type="Classification">
             <Description>

changes.rst

@@ -14,6 +14,10 @@ Version 3 (unreleased)
 
 .. rubric:: Alert Contents
 
+* Remove the ``skymap_png`` parameter from the VOEVents. The sky map plots take
+  longer to generate than the FITS files themselves, so they would have
+  needlessly delayed the preliminary alerts.
+
 .. rubric:: Sample Code
 
 Version 2 (2018-12-13)