Remove conditional removal of "Theme" settings menu option.

Previously the "Theme" settings menu option was removed if night mode
was not supported, but night mode is always supported as of
https://crrev.com/c/2298040. Removing this logic reduces the amount of
code that needs to be mirrored in the "Appearance" screen which will
serve as the future host for the "Theme" settings menu option.

Bug: 397775252
Change-Id: I77059e6c9201bf6583df70c728c61bd1dd42e8f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6283427
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1422309}