cleanup: remove enable-pci-guard-ui flag

Feature has been enabled by default since 2021-02-25 and flag
has been expired since M94.

Bug: None

Change-Id: Id60c488c2ee3cee765850fb4f8ba40a2e93a3bd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3445170


Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Jimmy Gong <jimmyxgong@chromium.org>
Cr-Commit-Position: refs/heads/main@{#975193}