benchmark cpu: drop KDF section

[mr-raj12]

Drops the KDF section from do_benchmark_cpu.

KDFs are supposed to be slow, that's the whole point. Timing them gives no useful signal, and the benchmark output already said so: "KDFs (slow is GOOD, use argon2!)".

Suggested in #9664 (review).

• Removed the KDF benchmark block (print header + for loop)
• Removed number_kdf and the FlexiKey import, both only used by that block

No logic changes.

Checklist

• PR is against master
• New code has tests
• Tests pass
• Commit message references related issue

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.62%. Comparing base (b0f3096) to head (fea52ef).
⚠️ Report is 22 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9665      +/-   ##
==========================================
+ Coverage   83.00%   83.62%   +0.61%     
==========================================
  Files          90       90              
  Lines       15549    15539      -10     
  Branches     2340     2337       -3     
==========================================
+ Hits        12907    12995      +88     
+ Misses       1887     1806      -81     
+ Partials      755      738      -17     

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[ThomasWaldmann]

Thanks!