Delete vercel.json from "yarn workspaces" example (#17263)

This PR removes "vercel.json" from the Yarn workspaces example. Since the release of [monorepos](https://vercel.com/blog/monorepos) support, there is no need to use the "builds" property or "vercel.json".

examples/with-yarn-workspaces/vercel.json

-{
-  "version": 2,
-  "builds": [{ "src": "packages/web-app/package.json", "use": "@now/next" }],
-  "rewrites": [{ "source": "/(.*)", "destination": "/packages/web-app/$1" }]
-}