Generalized indices: Revert original group_update PR

[J-Lentz]

Description
Revert the first attempt at generalized indices in mpp_group_update, so that we can get a clean performance baseline to test the new group_update PR against.

How Has This Been Tested?
Builds on the AMD box with ifx 2025.2.0 and mpp unit tests pass.

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