Clean up Incognito brand consistency for desktop flags.

This CL removes the following Incognito brand consistency feature flags:

a) IncognitoBrandConsistencyForDesktop
b) InheritNativeThemeFromParentWidget

The Incognito brand consistency project is default enabled since M97.

Bug: 1183334, 1288557
Change-Id: Id20e828479698e81be9c526d31d43011f6f669bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3404289


Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Rohit Agarwal <roagarwal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#962535}