treewide: purge maintainers.urkud

It's sad, but he's been inactive for the last five years.
Keeping such people in meta.maintainers is counter-productive.

pkgs/servers/sql/virtuoso/6.x.nix

@@ -40,6 +40,6 @@ stdenv.mkDerivation rec {
     description = "SQL/RDF database used by, e.g., KDE-nepomuk";
     homepage = http://virtuoso.openlinksw.com/dataspace/dav/wiki/Main/;
     platforms = platforms.linux;
-    maintainers = [ maintainers.urkud ];
+    maintainers = [ ];
   };
 }

pkgs/servers/sql/virtuoso/7.x.nix

@@ -31,6 +31,6 @@ stdenv.mkDerivation rec {
     homepage = http://virtuoso.openlinksw.com/dataspace/dav/wiki/Main/;
     #configure: The current version [...] can only be built on 64bit platforms
     platforms = [ "x86_64-linux" ];
-    maintainers = [ maintainers.urkud ];
+    maintainers = [ ];
   };
 }