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This is significantly faster (about 80x in some quick testing).

Also remove the now unused x509_util_mac IsSelfSigned function.

Bug: 410574
Change-Id: I60cfc40d5f5a5dab419566836555cb4828148c6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1570768Reviewed-by: Ryan Sleevi <rsleevi@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651774}