fix: remove unnecessary validation block

Although supplying incompatible network and subnetworks will not work,
it is not strictly required to supply just one or the other; we should
not introduce this change in behavior in 10.0 on a field that existed in
9.0.

modules/instance_template/variables.tf

@@ -230,12 +230,6 @@ variable "additional_networks" {
       subnetwork_range_name = string
     }))
   }))
-  validation {
-    condition = alltrue([
-      for ni in var.additional_networks : (ni.network == null) != (ni.subnetwork == null)
-    ])
-    error_message = "All additional network interfaces must define exactly one of \"network\" or \"subnetwork\"."
-  }
   validation {
     condition = alltrue([
       for ni in var.additional_networks : ni.nic_type == "GVNIC" || ni.nic_type == "VIRTIO_NET" || ni.nic_type == null