remove federation (#3794)

Signed-off-by: zounengren <zounengren@cmss.chinamobile.com>

man/coredns-kubernetes.7

@@ -257,26 +257,6 @@ cluster.local {
 .fi
 .RE
 
-.SH "FEDERATION"
-.PP
-The \fIkubernetes\fP plugin can be used in conjunction with the \fIfederation\fP plugin.  Using this
-feature enables serving federated domains from the Kubernetes clusters.
-
-.PP
-.RS
-
-.nf
-cluster.local {
-    federation {
-        prod prod.example.org
-        staging staging.example.org
-    }
-    kubernetes
-}
-
-.fi
-.RE
-
 .SH "WILDCARDS"
 .PP
 Some query labels accept a wildcard value to match any value.  If a label is a valid wildcard (*,