nucleati future cleanup: remove call_frm_zm_in

During the review of https://github.com/ESCOMP/atmospheric_physics/pull/411, @nusbaume originally wrote:

> You should certainly double-check me, but is call_frm_zm_in ever not .false. in CAM? If so then I might vote to remove it (or add a note in the "future work" issue to remove it later).

Start by reviewing pull request 411 and searching the repository for call_frm_zm_in and its callers. Check whether it can ever be true in CAM or related configurations; done means either removing the unused path safely or documenting why the cleanup must remain future work.

Written by the indexing model from the issue text.