Remove DataModel.open_asdf (deprecated since 3.0.1)

[emolter]

Closes #299

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

[emolter]

https://github.com/spacetelescope/RegressionTests/actions/runs/18232452558

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.12%. Comparing base (a2c393a) to head (1a75983).
⚠️ Report is 101 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #596      +/-   ##
==========================================
+ Coverage   90.08%   90.12%   +0.03%     
==========================================
  Files         111      111              
  Lines        5327     5317      -10     
==========================================
- Hits         4799     4792       -7     
+ Misses        528      525       -3     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.