Remove use_mpp_io from diag manager

[thomas-robinson]

Description
Removes the variable use_mpp_io from the diag manager.

Fixes # (issue)

How Has This Been Tested?
CI

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

[bensonr]

Looks good from what I can see.

[GFDL-Eric]

@thomas-robinson This doesn't actually have any duplication from #928 since I wasn't sure whether or not to get rid of the use_mpp_io namelist flag (if anyone still uses it). Of course, I could incorporate these changes into #928 if we feel like they should go together.

[thomas-robinson]

Thanks @GFDL-Eric I think you should keep #928 as is, and let this be part of the diag manager modernization. Then we can remove the rest of use_mpp_io in the "2022.05" end of year release.

[GFDL-Eric]

@thomas-robinson sounds good!