Remove use of MCT in GetGlobalValuesMod, decomp*, spmd* in cesm2_3 branch

[ekluzek]

CTSM went through a similar transformation, and we can likely use it as a guide to make this happen. But, the uses of MCT modules in these core SLIM modules needs to be removed. This is something that @mvertens can probably give some advice on. Since, the use of MCT is being deprecated in CESM, this is an important thing to do.

[mvertens]

@ekluzek - I'm not going to have a chance to look at this issue this week or next week. Once I get to Italy I'd be happy to look into this further.

[ekluzek]

That's OK, I think I'll be able to work around it for awhile. And then I should be able to use the CTSM example to get started on the change. I just think it will be likely that I'll have questions for you after you are in Italy and have started to settle in.