refactor(MeasureTheory): golf Mathlib/MeasureTheory/Measure/PreVariation

[yuanyi-350]

• refactors Mathlib/MeasureTheory/Measure/PreVariation by simplifying sum_le' and sum_le_preVariationFun_of_subset

Extracted from #38104

[github-actions]

PR summary 48f66c0814

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

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

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

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

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No changes to strong technical debt.

No changes to weak technical debt.

[yuanyi-350]

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

Results (seconds):

• sum_le': 0.503895 -> 0.130258 (-74.15%)
• sum_le_preVariationFun_of_subset: 0.119698 -> 0.044800 (-62.57%)

Overall:

• all compared declarations sped up.