Remove loops over calendar types where not needed

[mjs2369]

Description:

The subroutines set_calendar_type_string and get_calendar_string in the time_manager_mod have unnecessary loops over the number of max_types (calendar types). This PR removes these loops for improved code efficiency and readability

Fixes issue

fixes #653

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update

Documentation changes needed?

• [ ] My change requires a change to the documentation.
  • [ ] I have updated the documentation accordingly.

Tests

Ran model_mod_check and filter with print statements to ensure the correct calendar_type was being set/used

Checklist for merging

• [ ] Updated changelog entry
• [ ] Documentation updated
• [ ] Update conf.py

Checklist for release

• [ ] Merge into main
• [ ] Create release from the main branch with appropriate tag
• [ ] Delete feature-branch

Testing Datasets

• [ ] Dataset needed for testing available upon request
• [ ] Dataset download instructions included
• [x] No dataset needed

[hkershaw-brown]

For commits like these, where you've just missed something from the commit (removing the loop variable) and there aren't a bunch of people working with the pull request, you can rebase and squash the commits together. I'll go ahead an squish them.