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These methods (originally added in crrev.com/ef8f0ca9) were used
to mark which apps had been re-ordered on the NTP. The preference is
no longer checked anywhere.

Remove both the setter and getter, and add the preference to the list
of obsolete preferences to remove.

Bug: None
Change-Id: Ife36753fb52b5ede503fab02c839a3bf13fed824
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1836020Reviewed-by: Kelvin Jiang <kelvinjiang@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702488}