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 `num_iter` vectors.

#### Summary:

Removes one of the stan::mcmc::chains add methods that is no longer used by PyStan. It shouldn't be used in any event, as it assumes a different memory layout than is known to exist.

#### Intended Effect:

1. Clean up codebase

#### Side Effects:

None.

#### Documentation:

Not user-facing

#### Reviewer Suggestions:

Anyone.