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We are deprecating attestation methods by CryptohomeClient.

This CL also includes a simplied flow by consolidating
TpmAttestationDoesKeyExist and TpmAttestationGetCertificate, of which
results come from the same methods of attestation service anyway.

BUG=b:158955123
TEST=chromeos_unittests.

Change-Id: I29734d7a2b5546e60f9af599c656ade7dc6ea210
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2513949
Commit-Queue: Leo Lai <cylai@google.com>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823587}