ruby: remove myself as maintainer/CODEOWNER

It's been at least a year since I kept up to date with Ruby, and I
don't think I really have anything left to offer Nixpkgs in terms of
Ruby expertise.

pkgs/development/interpreters/ruby/rubygems/default.nix

@@ -30,6 +30,6 @@ stdenv.mkDerivation rec {
     description = "Package management framework for Ruby";
     homepage = "https://rubygems.org/";
     license = with licenses; [ mit /* or */ ruby ];
-    maintainers = with maintainers; [ qyliss zimbatm ];
+    maintainers = with maintainers; [ zimbatm ];
   };
 }