tcp: retire net.inet.tcp.tcp_require_unique_port

[glebius]

Summary

It was a safe belt just in case if the new port allocation
behaviour introduced in 25102351509 would cause a problem.

[markj]

tcp.4 references this sysctl, so needs to be updated as well.

[glebius]

Add man page diff.

[rscheff]

Also can not really think of any sane reason why you'd require unique ephemeral outgoing ports (outside of the local host). Quite the opposite, that restricts the total number of outgoing connections globally and not on a per-destination basis. Finally, this sysctl apparently was never mentioned anywhere (searchable) to be a workaround for any observed or perceived issue - while the original intent of the referenced patch is apparently working as expected.