Cleanup: Remove rice (reserved ice) from ZM

[jimmielin]

Issue Type

Code Clean-up

Issue Description

As discussed 1/20/2026 Projects Meeting: rice in ZM is unused and should be removed throughout.

Issue related to ESCOMP/atmospheric_physics#298 which brought up the question of rice in the ZM scheme.

Copying @adamrher @nusbaume @cacraigucar

Will this change answers?

No

Will you be implementing this yourself?

Yes