Remove logout button background color

This cl removes the background color of the logout button.

TrayBackgroundView sets a color to the layer on theme changed which
forces the layer to have the color. This should be removed because
the layer color should be set in UpdateBackground.

Bug: b:283319985
Change-Id: I7ca145fae0a09912ecf43c3bcac84caca41918b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4569264


Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Xiaodan Zhu <zxdan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149412}