Remove security check against secure connections

Changes to mitigate BLUFFS include a safeguard against downgrading from
Secure Connections mode.  However, this breaks connections with certain
carkits.  After changes to bond loss handling devices cannot reconnect
in these scenarios without reauthenticating, so the BLUFFS mitigation
is no longer relevant and can be backed out to fix carkit connections.

Bug: 390168795
Test: m libbluetooth
Ignore-AOSP-First: security
Tag: security
Flag: EXEMPT removes previously soaked check
Change-Id: I71015151be616580ff73f07cda0f0b43d44c8231