Remove unreachable special case

We had code to allow you to not specify an IPv6 machineNetwork value
on dual-stack AWS clusters, but we don't support dual-stack on AWS
anyway. And if we support it in the future, it will likely be after
improvements to IPv6 support on the AWS end, so this exception may no
longer make sense at that point anyway.

pkg/types/validation/installconfig.go

@@ -220,8 +220,6 @@ func validateNetworkingIPVersion(n *types.Networking, p *types.Platform) field.E
 		}
 		for k, v := range presence {
 			switch {
-			case k == "machineNetwork" && p.AWS != nil:
-				// AWS can default an ipv6 subnet
 			case v.IPv4 && !v.IPv6:
 				allErrs = append(allErrs, field.Invalid(field.NewPath("networking", k), strings.Join(ipnetworksToStrings(addresses[k]), ", "), "dual-stack IPv4/IPv6 requires an IPv6 network in this list"))
 			case !v.IPv4 && v.IPv6:

pkg/types/validation/installconfig_test.go

@@ -1112,16 +1112,6 @@ func TestValidateInstallConfig(t *testing.T) {
 			}(),
 			expectedError: `Invalid value: "10.0.0.0/16": dual-stack IPv4/IPv6 requires an IPv6 network in this list`,
 		},
-		{
-			name: "valid dual-stack configuration, machine has no IPv6 but is on AWS",
-			installConfig: func() *types.InstallConfig {
-				c := validInstallConfig()
-				c.Networking = validDualStackNetworkingConfig()
-				c.Networking.MachineNetwork = c.Networking.MachineNetwork[1:]
-				return c
-			}(),
-			expectedError: `Invalid value: "DualStack": dual-stack IPv4/IPv6 is not supported for this platform, specify only one type of address`,
-		},
 		{
 			name: "invalid dual-stack configuration, IPv6-primary",
 			installConfig: func() *types.InstallConfig {