remove TpmAttestationRegisterKey APIs

Now we are using AttestationClient directly instead of passing through
CryptohomeClient.

BUG=b:158955123
TEST=build ok.

Change-Id: I02646e7e9b7428a5f9ad41a07e80ef00ca5ef2e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505394
Commit-Queue: Leo Lai <cylai@google.com>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823187}