auto-task(QuantumInfo.ClassicalInfo.Capacity): minimize imports

[Alex-Zughaid]

Summary

Minimizes the imports of QuantumInfo/ClassicalInfo/Capacity.lean.

The file only defines Code, FixedLengthCode, and BlockCode structures over
List, Fin, and ℕ — it never uses anything from QuantumInfo.ClassicalInfo.Entropy
(entropy/probability/channel definitions), nor Fintype/Finset instances for Fin.
Replaced the three imports with the single, more basic import that the file actually
needs for its Fin block_in/Fin block_out usage:

• Removed QuantumInfo.ClassicalInfo.Entropy (unused)
• Removed Mathlib.Data.Finset.Fin (unused)
• Removed Mathlib.Data.Fintype.Fin (unused; replaced by the more basic Mathlib.Data.Fin.Basic)

Downstream impact

None. QuantumInfo.ClassicalInfo.Capacity is not imported by any other file in the
repo (its import in QuantumInfo.lean is commented out), so no other files needed
changes.

Verification

• lake build QuantumInfo.ClassicalInfo.Capacity — succeeds (510 jobs, down from 2216
  with the original imports)
• lake build QuantumInfo — succeeds (8733 jobs)
• lake build Physlib — succeeds (4448 jobs)

No sorrys or axioms were added; only import lines were touched.

[github-actions]

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[jstoobysmith]

Thanks Alex, I've approved this one and will merge it once the Linters finish.