bug: unused code in the mpas_atm model_mod needs removed

[mjs2369]

There is a subrountine latlon_to_xyz_on_plane() in the mpas_atm model_mod code that is no longer supported and unused (hasn't been touched in 10 years). It is also causing compiler warnings with cce on Derecho. So it's time to take it out.

Describe the bug

1. List the steps someone needs to take to reproduce the bug.
   compile mpas_atm on Derecho with cce
2. What was the expected outcome?
   compiles without warnings
3. What actually happened?
   warning messages outputted

Error Message

ftn-7212 ftn: WARNING LATLON_TO_XYZ_ON_PLANE, File = ../../../DART/models/mpas_atm/model_mod.f90, Line = 7209
Variable "intp" is used before it is defined.

Which model(s) are you working with?

mpas_atm

Screenshots

If applicable, add screenshots to help explain your problem.

Version of DART

Which version of DART are you using?
You can find the version using git describe --tags

v10.7.3

Have you modified the DART code?

No

Build information

Please describe:

1. The machine you are running on (e.g. windows laptop, NCAR supercomputer Cheyenne).
2. The compiler you are using (e.g. gnu, intel).

All systems and compilers