hedgewars: enable build on hydra

Was disabled when the package was marked as broken in
c333861954. It has since been un-broken,
but the hydraPlatforms exception was left presumably by mistake.

pkgs/games/hedgewars/default.nix

@@ -103,6 +103,5 @@ mkDerivation rec {
        all movement on the battlefield has ceased).'';
     maintainers = with maintainers; [ kragniz fpletz ];
     inherit (ghc.meta) platforms;
-    hydraPlatforms = [];
   };
 }