refactor(NumberTheory): golf Mathlib/NumberTheory/NumberField/Norm

[yuanyi-350]

• refactors NumberField/Norm by moving isUnit_norm_of_isGalois after dvd_norm and shortening its proof to a direct application of dvd_norm together with isUnit_of_dvd_unit

Extracted from #38144

[github-actions]

PR summary 4fceb4697e

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

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/pr_summary/declarations_diff.sh <optional_commit>

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

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

---

No changes to technical debt.

You can run this locally as

./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).

[yuanyi-350]

I ran a profiler comparison for the changed declarations in this PR.

Results (seconds):

• RingOfIntegers.isUnit_norm_of_isGalois: 0.181801 -> 0.023740 (-86.9%)

Overall:

• The changed proof elaborates substantially faster.

[riccardobrasca]

!radar

[leanprover-radar]

Benchmark results for f6e4989 against 4fceb46 are in. No significant results found. @riccardobrasca

• ✅ build//instructions: -14.1G (-0.01%)

Small changes (1✅)

• ✅ build/module/Mathlib.NumberTheory.NumberField.Norm//instructions: -1.1G (-6.95%)