fix: remove custom header from VMSS resize scenario test

The RP now gates VMSize resize by preview API version instead of AFEC
feature registration, so the AKSHTTPCustomFeatures header is no longer
needed.

Signed-off-by: wenhug <50309350+wenhug@users.noreply.github.com>

Author: wenhug

src/aks-preview/azext_aks_preview/tests/latest/test_aks_commands.py

@@ -23370,8 +23370,7 @@ def test_aks_nodepool_update_vmss_vm_size_resize(
         create_cmd = (
             "aks create --resource-group={resource_group} --name={name} "
             "--node-count=1 --node-vm-size Standard_D2s_v3 "
-            "--ssh-key-value={ssh_key_value} "
-            "--aks-custom-headers AKSHTTPCustomFeatures=Microsoft.ContainerService/AgentPoolVMSSResize"
+            "--ssh-key-value={ssh_key_value}"
         )
         self.cmd(
             create_cmd,
@@ -23385,8 +23384,7 @@ def test_aks_nodepool_update_vmss_vm_size_resize(
         update_cmd = (
             "aks nodepool update --resource-group={resource_group} "
             "--cluster-name={name} -n {nodepool_name} "
-            "--node-vm-size Standard_D4s_v3 "
-            "--aks-custom-headers AKSHTTPCustomFeatures=Microsoft.ContainerService/AgentPoolVMSSResize"
+            "--node-vm-size Standard_D4s_v3"
         )
         self.cmd(
             update_cmd,