refactor: golf Mathlib/MeasureTheory/Function/AEEqOfIntegral

[yuanyi-350]

This PR selectively ports part of #38108 and only migrates the changes in:

• Mathlib/MeasureTheory/Function/AEEqOfIntegral.lean

Concretely, this PR:

• rewrites Integrable.ae_eq_zero_of_forall_setIntegral_eq_zero to use AEFinStronglyMeasurable.ae_eq_zero_of_forall_setIntegral_eq_zero directly instead of going through Lp
• rewrites Integrable.ae_eq_of_forall_setIntegral_eq to use AEFinStronglyMeasurable.ae_eq_of_forall_setIntegral_eq directly instead of first reducing to the zero case via subtraction

This is a patch-only selective port prepared without running a build.

[github-actions]

PR summary 3853566136

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/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.

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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).

[grunweg]

Please follow the request made in the comments here and here. Thanks!