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The redesigned Clear All button is now shown at the top in the notification
stacking bar, so remove the bottom button.

BUG=952557

Change-Id: I4f471141094a01dc9cbc5b69f49cf87d69780097
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1659535Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Nik Bhagat <nikunjb@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670349}