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Over the years, the features controlled by some switches were removed
but the switches were left behind. Remove the unused switches, and add
a comment about how to avoid pitfalls in future cleanups.

Bug: 1130298
Change-Id: I7070fe6b8bd8044551e37979f0ebb7bf0e58fc75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421440
Commit-Queue: Eric Lawrence [MSFT] <ericlaw@microsoft.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809141}