chore: use ofNat more

[grunweg]

When a theorem statement contains both ofNat and OfNat.ofNat, replace the latter by the former: as eric-wieser says, these are mostly theorems that work forwards, but not backwards, with simp.

Very much not exhaustive, there are perhaps 160 instances in mathlib remaining.

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[grunweg]

@eric-wieser Does this match what you were thinking of?

[github-actions]

PR summary 47a30a39e3

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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No changes to technical debt.

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).