fix: remove stale slicing type ignore

[mkg0908]

Summary

• remove the unused type: ignore[misc] suppression from slicing()
• allow the mypy unused-ignore check to pass for the reported line

Tests

• .venv/bin/mypy src/alchemlyb/preprocessing/subsampling.py --ignore-missing-imports --disable-error-code no-any-return
• .venv/bin/python -m compileall -q src/alchemlyb/preprocessing/subsampling.py
• git diff --check

Closes #460

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (5c029f5) to head (d60b903).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #462   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          29       29           
  Lines        2446     2446           
  Branches      464      464           
=======================================
  Hits         2422     2422           
  Misses          2        2           
  Partials       22       22           

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[xiki-tempula]

Would it be better if you could fix the other mypy and ruff issue, please? Thanks!