[Merged by Bors] - refactor(Algebra/Group/Subgroup/Basic): clean up normalizer lemmas

[tb65536]

This PR cleans up several normalizer lemmas in Algebra/Group/Subgroup/Basic. Several lemmas had unnecessary assumptions or unnecessarily complicated proofs.

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[github-actions]

PR summary 8bbe260b53

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

+ _root_.AddSubgroup.comap_normalizer_eq_of_injective_of_le_range
+ comap_normalizer_eq_of_le_range
+ inf_normalizer_le_normalizer_inf
+ normal_subgroupOf_iff_le_normalizer
+ normal_subgroupOf_iff_le_normalizer_inf

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).

[Paul-Lez]

I believe this already exists as sup_idem (and similarly for the inf lemma below).

[tb65536]

Ah, thanks!

[jcommelin]

LGTM, mod the comment about sup_idem.

bors d+

[mathlib-bors]

✌️ tb65536 can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

[tb65536]

bors r+

[mathlib-bors]

Pull request successfully merged into master.

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