TST: Delete unused jwst/lib/tests/stp_v1.py

[pllim]

This PR addresses #9283 (comment) cc @stscieisenhamer

Tasks

• If you have a specific reviewer in mind, tag them.
• add a build milestone, i.e. Build 12.0 (use the latest build if not sure)
• Does this PR change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see changelog readme for instructions)
  • update or add relevant tests
  • update relevant docstrings and / or docs/ page
  • start a regression test and include a link to the running job (click here for instructions) https://github.com/spacetelescope/RegressionTests/actions/runs/14315202085
    • Do truth files need to be updated ("okified")?
      • after the reviewer has approved these changes, run okify_regtests to update the truth files
• if a JIRA ticket exists, make sure it is resolved properly

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.18%. Comparing base (4a08d6c) to head (bf47c23).
Report is 685 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9366   +/-   ##
=======================================
  Coverage   75.18%   75.18%           
=======================================
  Files         368      368           
  Lines       36837    36837           
=======================================
  Hits        27696    27696           
  Misses       9141     9141           

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[melanieclarke]

I think this is okay to remove. We can restore it from git history if it turns out to have a secret use somewhere.