remove unused sphinx-asdf

[braingram]

sphinx-asdf is listed as a required dependency for docs. It is unused by the documentation (it handles rendering schemas so is useful for RAD but not roman_datamodels).

This PR removes sphinx-asdf as a dependency.

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 97.57%. Comparing base (087a60d) to head (ec687b6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #326      +/-   ##
==========================================
+ Coverage   97.56%   97.57%   +0.01%     
==========================================
  Files          30       31       +1     
  Lines        2788     2808      +20     
==========================================
+ Hits         2720     2740      +20     
  Misses         68       68              

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[PaulHuwe]

LGTM