Remove unused `check_parallel_system` function

[altheaden]

This function was meant to be removed in a prior pull request (https://github.com/E3SM-Project/polaris/pull/76), but was missed when rebasing from Compass.

Checklist

• [x] API documentation in the Developer's Guide (api.md) has any new or modified class, method and/or functions listed
• [x] Testing comment in the PR documents testing used to verify the changes

[xylar]

Testing

I ran the cosine bell test suite on Chrysalis with this branch and both the QU and Icos tests passed. (I didn't do a baseline comparison because I think that's overkill in this case.)

[xylar]

Thanks, @altheaden!