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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=unit_tests.

Change-Id: Ib1f9de5a6db041e8c3a86c5d90a0f55b8e4d8285
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515800
Commit-Queue: Leo Lai <cylai@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823889}