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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.66%. Comparing base (087a60d) to head (4e81378).
Report is 60 commits behind head on main.

Additional details and impacted files

@@            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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.