obsolete: clarify users in markerusers() never contain None

obsolete: clarify users in markerusers() never contain None

--- a/mercurial/obsutil.py	Thu Jul 12 23:07:29 2018 +0900
+++ b/mercurial/obsutil.py	Sun Jul 15 18:22:40 2018 +0900
@@ -822,7 +822,7 @@
     """ Returns a sorted list of markers users without duplicates
     """
     markersmeta = [dict(m[3]) for m in markers]
-    users = set(meta.get('user') for meta in markersmeta if meta.get('user'))
+    users = set(meta['user'] for meta in markersmeta if meta.get('user'))
 
     return sorted(users)