fix: remove old version @antv/util to reduce size

Author: hustcc

package.json

@@ -34,7 +34,6 @@
     "silent": false
   },
   "dependencies": {
-    "@antv/util": "^2.0.7"
   },
   "devDependencies": {
     "@babel/cli": "^7.0.0",

src/util.js

@@ -13,6 +13,6 @@ function getHeight(preNode, node, align, heightField = 'height') {
 }
 
 module.exports = {
-  assign: mix,
+  assign: Object.assign,
   getHeight
 };