Update expressroute-howto-gateway-migration-portal.md

remove all info about adding second prefix

Author: MekaylaMoore

articles/expressroute/expressroute-howto-gateway-migration-portal.md

@@ -29,28 +29,6 @@ The following SKUs are available for ExpressRoute virtual network gateways:
 
 - Review the [Gateway migration](gateway-migration.md) article before you begin.
 - You must have an existing [ExpressRoute Virtual network gateway](expressroute-howto-add-gateway-portal-resource-manager.md) in your Azure subscription.
-- A second prefix is required for the gateway subnet. If you have only one prefix, you can add a second prefix by following the steps in the [Add a second prefix to the gateway subnet](#add-a-second-prefix-to-the-gateway-subnet) section.
-  
-## Add a second prefix to the gateway subnet
-
-The gateway subnet needs two or more address prefixes for migration. If you have only one prefix, you can add a second prefix by following these steps.
-
-1. First, update the `Az.Network` module to the latest version by running this PowerShell command:
-
-    ```powershell-interactive
-    Update-Module -Name Az.Network -Force
-    ```
-
-1. Then, add a second prefix to the **GatewaySubnet** by running these PowerShell commands:
-
-    ```powershell-interactive
-    $vnet = Get-AzVirtualNetwork -Name $vnetName -ResourceGroupName $resourceGroup
-    $subnet = Get-AzVirtualNetworkSubnetConfig -Name GatewaySubnet -VirtualNetwork $vnet
-    $prefix = "Enter new prefix"
-    $subnet.AddressPrefix.Add($prefix)
-    Set-AzVirtualNetworkSubnetConfig -Name GatewaySubnet -VirtualNetwork $vnet -AddressPrefix $subnet.AddressPrefix
-    Set-AzVirtualNetwork -VirtualNetwork $vnet
-    ```
 
 ## Migrate to a new gateway in Azure portal
 
@@ -98,4 +76,4 @@ Here are the steps to migrate to a new gateway in Azure portal.
 ## Next steps
 
 * Learn more about [designing for high availability](designing-for-high-availability-with-expressroute.md).
-* Plan for [disaster recovery](designing-for-disaster-recovery-with-expressroute-privatepeering.md) and [using VPN as a backup](use-s2s-vpn-as-backup-for-expressroute-privatepeering.md).
+* Plan for [disaster recovery](designing-for-disaster-recovery-with-expressroute-privatepeering.md) and [using VPN as a backup](use-s2s-vpn-as-backup-for-expressroute-privatepeering.md).