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Clients, starting at M-72, no longer support top-level hint
optimizations for the existing optimization types, so the code
supporting them is being removed.

The server may add new top-level optimization types in the future,
so the proto field is not being deprecated.

Bug: 917189

Change-Id: Ib6a1bb96958ff96dfd65921d9afd4b8bac8bf39c
Reviewed-on: https://chromium-review.googlesource.com/c/1387950
Commit-Queue: Jered Gray <jegray@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618585}