chore(Algebra/DirectSum): remove use of erw in mul_eq_dfinsuppSum

[euprunin]

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[github-actions]

PR summary 89ec9c848c

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.

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Decrease in tech debt: (relative, absolute) = (1.00, 0.00)

Current number	Change	Type
509	-1	porting notes
707	-1	erw

Current commit 6820ad62db
Reference commit 89ec9c848c

You can run this locally as

./scripts/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).

[erdOne]

Thanks!
maintainer merge

[github-actions]

🚀 Pull request has been placed on the maintainer queue by erdOne.

[jcommelin]

Thanks 🎉

bors merge

[mathlib-bors]

Merge conflict.

[mathlib4-merge-conflict-bot]

This pull request has conflicts, please merge master and resolve them.