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/tools/misc/convmv/default.nix

@@ -25,6 +25,6 @@ stdenv.mkDerivation rec {
   meta = with stdenv.lib; {
     description = "Converts filenames from one encoding to another";
     platforms = platforms.linux ++ platforms.freebsd ++ platforms.cygwin;
-    maintainers = [ maintainers.urkud ];
+    maintainers = [ ];
   };
 }