remove debug check no longer needed for control fields

[Yurlungur]

PR Summary

What it says on the tin. Sparse control fields mean mean that a sparse variable may not be allocated when it receives boundary data, if it's controlled by another variable. We just missed the removal of this debug check when that feature was made robust.

PR Checklist

• Code passes cpplint
• New features are documented.
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• Changes are summarized in CHANGELOG.md
• CI has been triggered on Darwin for performance regression tests.
• Docs build
• (@lanl.gov employees) Update copyright on changed files

[jdolence]

LGTM. Let's get this merged ASAP.

[lroberts36]

LGTM, thanks for catching that!