Remove unused add method accepting vectors

This function is no longer used by PyStan and it is, moreover, designed
incorrectly. In RStan and PyStan chains are effectively stored in "row order" as
each parameter has its own vector<double> of size `num_iter`. This function
assumes this arrangement is transposed, that one has has `num_iter` vectors.
Merge pull request #1020 from ariddell/feature/remove-chains-add-method

Remove unused add method accepting vectors