chore: prefer classical over open scoped Classical in proofs

[felixpernegger]

In #41423 it came up that it is preferable to use classical in proofs instead of open (scoped) Classical, as its slightly easier to maintain. This PR replaces all such occurences where it can be reasonably done.

This is likely my last PR about this topic.

(I used Claude to fix some git issue, which is why it shows up in the commit)

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[github-actions]

PR summary 1d4cde4641

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff (regex)

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit 1d4cde4).

• +0 new declarations
• −0 removed declarations

No declaration differences.

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No changes to strong technical debt.

No changes to weak technical debt.

Current commit 1d4cde4641
Reference commit 2d00c9242a

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[felixpernegger]

!bench

[leanprover-radar]

Benchmark results for 1d4cde4 against 2d00c92 are in. No significant results found. @felixpernegger

• ✅ build//instructions: -81.2G (-0.05%)

No significant changes detected.