hide the --enable-managed-system-pool optoin for now

Author: Hao Yuan

src/aks-preview/azext_aks_preview/_help.py

@@ -660,10 +660,6 @@
         - name: --vm-sizes
           type: string
           short-summary: Comma-separated list of sizes. Must use VirtualMachines agent pool type.
-        - name: --enable-managed-system-pool
-          type: bool
-          short-summary: Create a default ManagedSystem mode that is fully managed by AKS.
-          long-summary: When set, the default system node pool is created with ManagedSystem mode, where all properties except name and mode are managed by AKS. Learn more at https://aka.ms/aks/nodepool/mode.
         - name: --enable-upstream-kubescheduler-user-configuration
           type: bool
           short-summary: Enable user-defined scheduler configuration for kube-scheduler upstream on the cluster