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This CL deletes 1+ year old code for removing old prefs.

Change-Id: Id56e4fc76f342933cef3116222587abe033f142f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611500
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840907}