const gcs = require('@google-cloud/storage')();
const functions = require('firebase-functions');
...
  const bucket = gcs.bucket(functions.config().firebase.storageBucket);

  return bucket.deleteFiles({
    prefix: `users/${userId}/`
  }, function(err) {
    if (err) {
      console.log(err);
    } else {
      console.log(`All the Firebase Storage files in users/${userId}/ have been deleted`);
    }
  });