configuration-common.nix: drop obsolete override

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

@@ -46,9 +46,6 @@ self: super: {
   # 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; };
 
-  # help blake finding its native library
-  hs-blake2 = super.hs-blake2.override { b2 = pkgs.libb2; };
-
   # Use the default version of mysql to build this package (which is actually mariadb).
   mysql = super.mysql.override { mysql = pkgs.mysql.lib; };