Revert "Add a test for unthemed browser frames"

This reverts commit 5fb39fe5.

Reason for revert: Suspicious about test failures at All/BrowserNonClientFrameViewRefreshedBrowserTest.UnthemedFrameColor
https://ci.chromium.org/p/chromium/builders/ci/linux-chromeos-rel/66684

Original change's description:
> Add a test for unthemed browser frames
>
> Guard against accidental theming of the browser frame in ChromeOS.
>
> Follow up for https://crrev.com/7b03f492c
>
> Change-Id: I32fa174ebc350f65558a358342124eae1c16d0b4
> Bug: b:282841014
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4538684


> Reviewed-by: Emily Shack <emshack@chromium.org>
> Commit-Queue: Sean Kau <skau@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1147407}

Bug: b:282841014
Change-Id: I0c46057a2bd201e8bf3709421bd6347b941ea061
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4554697
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Yuki Shiino <yukishiino@chromium.org>
Auto-Submit: Yuki Shiino <yukishiino@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1147616}