remove unused function from mpp test

[rem1776]

Description
Removes an unused legacy function from test_mpp_gatscat.F90. The function uses SIZE calls on unallocated variables which fails to compile with ifort 2024.01.

Fixes #1504

How Has This Been Tested?
make distcheck on amd box with oneapi 2024.01

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes