Revert "Extensions: CodeHealth: Prefer CHECK_IS_TEST() for test-only methods"

This reverts commit 6d3f5d32c34cbaa88320f6d5475343d659dc829f.

Reason for revert: CHECK_IS_TEST is unnecessary within test-only functions as presubmits already block calling them from production code.

Bug: chromium:346952289
Original change's description:
> Extensions: CodeHealth: Prefer CHECK_IS_TEST() for test-only methods
>
> Reference:
> https://chromium.googlesource.com/chromium/src/+/main/styleguide/c++/c++.md#test_only-code-paths-in-production-code
>
> This CL was uploaded by git cl split.
>
> R=mcnee@chromium.org
>
> Bug: chromium:346952289
> Change-Id: I7a58148940cae4dcfb07cfa832c8eb0aa8bbce86
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6477611
> Auto-Submit: Solomon Kinard <solomonkinard@chromium.org>
> Commit-Queue: Kevin McNee <mcnee@chromium.org>
> Reviewed-by: Kevin McNee <mcnee@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1450131}

Bug: chromium:346952289
Change-Id: Id74f222dd72f44e6461839e17e32110ebf02cfb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6479001
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Kevin McNee <mcnee@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1450265}