OCPBUGS-43736: Remove erroneous field

- https://issues.redhat.com/browse/OCPBUGS-43736

Author: jab-rh

modules/nw-operator-cr.adoc

@@ -417,7 +417,6 @@ spec:
   serviceNetwork:
   - 172.30.0.0/16
   networkType: OVNKubernetes
-      clusterNetworkMTU: 8900
 ----
 endif::operator[]
 endif::post-install-network-configuration[]