Revert "[CrOS Customization] Update app icons"

This reverts commit dc1d693f33.

Reason for revert: No longer wanted in M-87

Original change's description:
> [CrOS Customization] Update app icons
>
> Includes icons for the Settings, Wallpapers, and Keyboard Shortcuts
> apps.
>
> Screenshot: https://screenshot.googleplex.com/7KdMiBkMFH7fuxh.png
>
> Change-Id: I6cf1d45d78f36caeb1429629cc0067b1cbd57daa
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429395
> Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
> Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
> Commit-Queue: James Cook <jamescook@chromium.org>
> Reviewed-by: James Cook <jamescook@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#810376}

Bug: 1138609
Change-Id: Iaa04c22814b3d8cd1ddc06ceb7a7beb14ad8d030
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2472346
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817236}