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/development/libraries/libssh2/default.nix

@@ -31,6 +31,6 @@ stdenv.mkDerivation rec {
     description = "A client-side C library implementing the SSH2 protocol";
     homepage = http://www.libssh2.org;
     platforms = stdenv.lib.platforms.all;
-    maintainers = [ stdenv.lib.maintainers.urkud ];
+    maintainers = [ ];
   };
 }