sshd: remove blacklist call from grace_alarm_timer

Under certain circumstances it may call log(3), which is not async-
signal-safe.

For now just remove the blacklist integration from this path, which
means that blacklistd will not detect and firewall hosts that establish
a connection but do nothing further.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46203