chore: cleanup xss example (#1237)

remove express middleware from the example

Author: acao

packages/graphql-playground-html/examples/xss-attack/package.json

@@ -10,7 +10,6 @@
   "author": "",
   "license": "ISC",
   "dependencies": {
-    "apollo-server-express": "^2.11.0",
     "express": "^4.17.1",
     "graphql": "^15.0.0",
     "graphql-playground-html": "1.6.20",