Cros: Remove device wallpaper policy related dead code.

This is a piece of remaining dead code after the changes in
crbug.com/705028: Device policy controlled wallpaper should only show up
in the login screen and should not affect user's session. Clean it up.

Bug: 705028
Change-Id: I08728901f36f7c3a2b768321eea733be49f16fff
Reviewed-on: https://chromium-review.googlesource.com/624184
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496842}