haskell-hruby: remove obsolete override

Closes https://github.com/NixOS/nixpkgs/pull/13920.

pkgs/development/haskell-modules/configuration-common.nix

@@ -44,9 +44,6 @@ self: super: {
   statistics = dontCheck super.statistics;
   c2hs = if pkgs.stdenv.isDarwin then dontCheck super.c2hs else super.c2hs;
 
-  # The package doesn't compile with ruby 1.9, which is our default at the moment.
-  hruby = super.hruby.override { ruby = pkgs.ruby_2_1; };
-
   # Use the default version of mysql to build this package (which is actually mariadb).
   mysql = super.mysql.override { mysql = pkgs.mysql.lib; };