[DF][NFC] Simplify RAction.hxx:InitColumnReaders
[DF][NFC] Simplify RAction.hxx:InitColumnReaders

It does not need the index_sequence<S...> parameter, and the less
variadic template parameters, the better.

root-mirror/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,karies/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,olifre/root,karies/root,karies/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,karies/root,olifre/root