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

(cherry picked from commit 2739a6845031e69be7c03461a9335d8bbb9f59bd)
(cherry picked from commit 3d3bae9b95388169d396adc8007585699c5a23e0)

Approved by: so