Remove duplicate available_platforms fixture from septop slow tests

[IAlibay]

Removed the 'available_platforms' fixture from the test.

Checklist

• Added a news entry

Developers certificate of origin

• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[github-actions]

No API break detected ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.41%. Comparing base (e14f2de) to head (1b147d4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1586      +/-   ##
==========================================
- Coverage   95.44%   93.41%   -2.04%     
==========================================
  Files         174      174              
  Lines       14545    14542       -3     
==========================================
- Hits        13883    13584     -299     
- Misses        662      958     +296     

Flag	Coverage Δ
fast-tests	93.41% <ø> (?)
slow-tests	?

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