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Since we're not using any of the notification permission prompt flavors,
this CL cleans up all the code that was introduced to experiment with
them.

Bug: 1029718
Change-Id: I40e29f1ff5efad30a76b532ff2184561a06d4882
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1949363
Commit-Queue: Andy Paicu <andypaicu@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734947}