Fix leak with CertVerifier::Config additional_untrusted_authorities

MultiThreadedCertVerifier::SetConfig was just adding new certs onto the
end of `temp_certs_` and never removing the old ones, so `temp_certs_`
could grow without bound.

Also remove the DCHECK. It is valid and expected that there can be
certificates that are accepted by X509Certificate but not by
CERTCertificate.

Also clarify the variable naming a bit.

Change-Id: Ifbbda388a333a87e83bb65babbff0082747c3da3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3611965
Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#996919}