Cert Management UI V2: remove platform user certs section on ChromeOS

On ChromeOS the user certs from the "platform" (NSS) get migrated into
the ServerCertificateDatabase as part of the launch, after which NSS is
not used for cert verification at all. Showing the no longer used NSS
certs in the UI does not make sense, therefore remove the section
completely.

Bug: 40928765
Change-Id: If564e26e503e61a17060e7c61544eb340c69baaa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6196975
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Hubert Chao <hchao@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1412014}