build: resolve dependency cycle

Author: meteorlxy

packages/@vuepress/markdown/package.json

@@ -30,9 +30,6 @@
     "markdown-it-table-of-contents": "^0.4.0",
     "prismjs": "^1.13.0"
   },
-  "devDependencies": {
-    "@vuepress/test-utils": "^1.4.0"
-  },
   "publishConfig": {
     "access": "public"
   },