Remove NativeTheme::GetInstanceForDarkUI().

This is never used; the only calls are attempts to notify it of changes.

Bug: 431953872
Change-Id: I20644a44f8082ef743e2f210e07691f48500d72c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6851629
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Tom Lukaszewicz <tluk@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1502606}