Change default mapnames from "idmap" to "unset"

[billsacks]

Based on discussion with @mvertens (in the context of https://github.com/ESMCI/cime/issues/3962): The default mapnames in this block of config_component.xml should be changed from idmap to unset:

https://github.com/ESCOMP/CMEPS/blob/cf0d47ba7780783bdc812fb420322ce1b782b810/cime_config/config_component.xml#L1354-L1553

The problem with having the default be idmap is: For a non-required gridmap (e.g., GLC2ICE_RMAPNAME), if mapping files are not explicitly set for the given grid, then they remain at idmap. The correct thing in this situation (according to @mvertens) would be for them to be unset in this situation.

I don't think this actually needs to be addressed for the sake of https://github.com/ESMCI/cime/issues/3962, so fixing this isn't urgent as far as I'm concerned. At first I thought I'd combine a fix for this cmeps issue with a fix for that cime issue, but I think it would actually be best to keep them separate: this cmeps issue will cause some namelist changes, whereas I'd like to ensure that the extensive code changes for https://github.com/ESMCI/cime/issues/3962 do not cause any namelist changes.

The changes to fix this cmeps issue should be simple; the hard part for me is knowing what testing to do in order to verify that this change doesn't break anything. @mvertens, can you provide any guidance here?