Revert "Allow api_gateway_gateway to change (#4557) (#3022)"

This reverts commit 51e758f.

Author: melinath

.changelog/4557.txt

@@ -45,6 +45,7 @@ func resourceApiGatewayGateway() *schema.Resource {
 			"api_config": {
 				Type:             schema.TypeString,
 				Required:         true,
+				ForceNew:         true,
 				DiffSuppressFunc: compareResourceNames,
 				Description:      `Resource name of the API Config for this Gateway. Format: projects/{project}/locations/global/apis/{api}/configs/{apiConfig}`,
 			},
@@ -255,12 +256,6 @@ func resourceApiGatewayGatewayUpdate(d *schema.ResourceData, meta interface{}) e
 	} else if v, ok := d.GetOkExists("display_name"); !isEmptyValue(reflect.ValueOf(v)) && (ok || !reflect.DeepEqual(v, displayNameProp)) {
 		obj["displayName"] = displayNameProp
 	}
-	apiConfigProp, err := expandApiGatewayGatewayApiConfig(d.Get("api_config"), d, config)
-	if err != nil {
-		return err
-	} else if v, ok := d.GetOkExists("api_config"); !isEmptyValue(reflect.ValueOf(v)) && (ok || !reflect.DeepEqual(v, apiConfigProp)) {
-		obj["apiConfig"] = apiConfigProp
-	}
 	labelsProp, err := expandApiGatewayGatewayLabels(d.Get("labels"), d, config)
 	if err != nil {
 		return err
@@ -280,10 +275,6 @@ func resourceApiGatewayGatewayUpdate(d *schema.ResourceData, meta interface{}) e
 		updateMask = append(updateMask, "displayName")
 	}
 
-	if d.HasChange("api_config") {
-		updateMask = append(updateMask, "apiConfig")
-	}
-
 	if d.HasChange("labels") {
 		updateMask = append(updateMask, "labels")
 	}