Remove check of Faravahar's IPv6 address on the relay post-install page

Section 7 on https://community.torproject.org/relay/setup/post-install/ provides some IPv6 check command. However, executing the command results in an error as Faravahar is gone. Thus, we should at least update that command to exclude Faravahar's IPv6 address.

Noted in our forum: https://forum.torproject.net/t/directory-authority-ipv6-2607154-3-unreachable/7222