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The Spelling Service ("Enhanced Spell Check") was migrated from a
JSON-RPC endpoint to a REST endpoint. This migration was rolled out
as a finch experiment to ensure nothing had regressed.

Now that everything has been working well for a few months, it's time to
clean up everything related to the Finch experiment and the old JSON-RPC
endpoint.

Metrics about the new endpoint:
https://uma.googleplex.com/histograms?sid=53ae9a4522ca75cb8f945bf4422baafe

Migration bug:
https://bugs.chromium.org/p/chromium/issues/detail?id=943108

Migration mini design doc:
https://docs.google.com/document/d/1tmqz3QTF-lhbbkBxqOSYMdNZ-OOhlMIhb8hczhbZVhQ

Bug: 943108
Change-Id: If46ffbbbdf956f7ed7000ade6c3bd9c5252361f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1932095Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#719592}