remove unnecessary generic_io usage

[braingram]

Remove an unnecessary usage of asdf.generic_io (there are developing plans to remove this submodule as it's largely unnecessary as seen in this PR).

Regression tests: https://github.com/spacetelescope/RegressionTests/actions/runs/25395302167
all pass

Tasks

• update or add relevant tests
• update relevant docstrings and / or docs/ page
• Does this PR change any API used downstream? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • run jwst regression tests with this branch installed ("git+https://github.com/<fork>/stdatamodels@<branch>")

news fragment change types...

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: fixes an issue
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.misc.rst: infrastructure or miscellaneous change

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.75%. Comparing base (9454993) to head (f75d362).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #729      +/-   ##
==========================================
- Coverage   90.76%   90.75%   -0.01%     
==========================================
  Files          99       99              
  Lines        4578     4577       -1     
==========================================
- Hits         4155     4154       -1     
  Misses        423      423              

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[emolter]

LGTM