ssh: remove unused variable

This was introduced in 03f6c5cd93ec, which added use of
sysctl net.inet.ip.portrange.reservedhigh instead of IPPORT_RESERVED,
but it appears the rest of that change was lost in some subsequent
update.

The change should probably be restored, but until then there is no
reason to leave an unused variable around.

MFC after: 3 days
Sponsored by: The FreeBSD Foundation