chore: remove some unused variables

[adomani]

Found by the unused variable command linter (in progress).

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[github-actions]

PR summary 99275e79b7

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

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

You can run this locally as follows

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

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

[Ruben-VandeVelde]

Maybe

Suggested change

	@[simp] lemma mul_div_cancel_left₀ {a : M₀} (b : M₀) (ha : a ≠ 0) : a * b / a = b := by
	@[simp] lemma mul_div_cancel_left₀ (b : M₀) {a : M₀} (ha : a ≠ 0) : a * b / a = b := by

Though on the other hand, maybe we just shouldn't make these changes here

[adomani]

I'd prefer to leave these changes to a separate PR, since I would not want to mix variable removals with variable order changes (at least if I notice that I am doing a reorder!).