Inline case_name variable

[StephanDeHoop]

Issue
Resolves #9252

Approach
We remove CASE_NAME so this might be confusing (and it's unnecessary)

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.86%. Comparing base (2b41fe2) to head (6471d2d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/everest/export.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9594      +/-   ##
==========================================
+ Coverage   91.85%   91.86%   +0.01%     
==========================================
  Files         433      433              
  Lines       26775    26773       -2     
==========================================
+ Hits        24594    24595       +1     
+ Misses       2181     2178       -3     

Flag	Coverage Δ
cli-tests	39.75% <ø> (ø)
everest-models-test	34.58% <0.00%> (+<0.01%)	⬆️
gui-tests	72.12% <ø> (ø)
integration-test	37.19% <50.00%> (+<0.01%)	⬆️
performance-tests	51.95% <ø> (-0.03%)	⬇️
test	40.43% <50.00%> (-0.03%)	⬇️
unit-tests	74.19% <0.00%> (+0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #9594 will not alter performance

_{Comparing StephanDeHoop:remove_casename_variable (6471d2d) with main (2b41fe2)}

Summary

✅ 22 untouched benchmarks