wallpaper: remove accessor GetWallpaperColors

Nothing uses the returned values. As part of dark/light launch,
wallpaper color is now used in DarkLightModeController
and propagates colors to ChromeOS UI via AshColorProvider.

TEST=ash_unittests --gtest_filter="*AppListController*"
TEST=ash_unittests --gtest_filter="*WallpaperController*"
BUG=none

Change-Id: I0b84769c1f64f982e9048eb0851dd76cb2c00e6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3939994
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Jeffrey Young <cowmoo@chromium.org>
Reviewed-by: Jason Thai <jasontt@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1057609}