Air density at interfaces repeated computation

From https://github.com/ESCOMP/atmospheric_physics/pull/292 review

@nusbaume wrote:
> Also I might make a comment here noting that eventually this can be replaced with a more generic variable, as there is nothing gravity-wave-specific about how it is being calculated in the interstitial scheme:

Right now `rhoi` is computed in both `gravity_wave_drag_prepare_profiles_run` and in the diffusion solver `vertical_diffusion_interpolate_to_interfaces` (as `rhoi_dry`)