remove deprecated skip_fits_update

[braingram]

Resolves JP-nnnn
Resolves RCAL-nnnn

Closes #

This PR addresses ...

Regtests (with hash ignored): https://github.com/spacetelescope/RegressionTests/actions/runs/12998390810

Blocked by #381

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 78.31%. Comparing base (4e83608) to head (00cd67c).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #380      +/-   ##
==========================================
+ Coverage   78.16%   78.31%   +0.15%     
==========================================
  Files         115      115              
  Lines        5144     5129      -15     
==========================================
- Hits         4021     4017       -4     
+ Misses       1123     1112      -11     

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