Let the Vercel CLI do the build

[As per the Vercel CLI docs as of March 2022](https://vercel.com/docs/cli#project-configuration), it is no longer recommended to provide the `builds` field ourselves. Instead, the `functions` field should be used, but only when a Serverless Function is due to be deployed. For all other cases, the Vercel CLI should handle the build by itself, given that it knows what steps to run based on the project id (or at least this is what I understand by reading the docs, someone please correct me if I'm wrong).

Whatever the case, it's clear that `builds` and `routes` should not be used anymore: