remove use_mpp_io namelist from FMS source code

[vithikashah001]

Description
removes the deprecated use_mpp_io namelist parameter and associated code blocks from the FMS source code.

Fixes #1881

How Has This Been Tested?

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

[uramirez8707]

One small change, but it looks good otherwise. Thanks @vithikashah001