flags: Clean up allow-eap-default-cas-without-subject-verification

Certificate based EAP auth methods are required to specify subject or
domain match options, if the EAP config uses the default CA certs,
since version 112.

This flag does not affect managed networks which can continue using
EAP with default CA certs without specifying any match option to verify
the identity of the server.

Bug: b/40284171
Change-Id: I29225534e09db2bcdc4623da6047ff9c9f209d74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5387679


Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Andreea Costinas <acostinas@google.com>
Cr-Commit-Position: refs/heads/main@{#1281703}