Do not set the MTU when using NetworkAttachementDefinition

The static MTU can be incorrect in some clusters, instead
rely on the fact that when the MTU is left unset OVN-Kubernetes
will use a default equal to the clusterNetworkMTU.
https://github.com/ovn-kubernetes/ovn-kubernetes/blob/1df91ecad2dc2e283dd4106e2e5069aff9eb1eb5/go-controller/pkg/config/cni.go#L103

Signed-off-by: Patryk Diak <[email protected]>

Author: kyrtapz

test/extended/networking/network_segmentation.go

@@ -1919,7 +1919,6 @@ func generateNAD(config networkAttachmentConfig) *nadapi.NetworkAttachmentDefini
         "topology":%q,
         "subnets": %q,
         "excludeSubnets": %q,
-        "mtu": 1300,
         "netAttachDefName": %q,
         "vlanID": %d,
         "allowPersistentIPs": %t,