remove unreachable code due to new asdf version requirement

[braingram]

Closes #364

This PR removes code that is now unreachable since the minimum asdf version is >3.dev.

Checklist

• [ ] Added entry in CHANGES.rst under the corresponding subsection
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/romancal/XXX/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.66%. Comparing base (087a60d) to head (4e81378). Report is 30 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #365 +/- ## ========================================== + Coverage 97.56% 97.66% +0.09% ========================================== Files 30 37 +7 Lines 2788 3334 +546 ========================================== + Hits 2720 3256 +536 - Misses 68 78 +10 ```

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