Remove reference to dns_v4_first in docs

Author: m-bull

doc/source/configuration/walled-garden.rst

@@ -45,14 +45,6 @@ seed:
    # HTTP(S) requests from control plane hosts.
    seed_squid_container_enabled: true
 
-In some environments we have found that squid’s preference for IPv6 can
-cause problems. It can be forced to prefer IPv4, by adding the following
-in ``etc/kayobe/containers/squid_proxy/squid.conf``:
-
-.. code::
-
-   dns_v4_first on
-
 In ``etc/kayobe/inventory/group_vars/overcloud/proxy`` (and any other
 groups that need to use the proxy), configure overcloud hosts to use the
 proxy: