Remove preview routes since next-drupal shall not get used

pages/api/exit-preview.tsx

-import { NextApiResponse } from "next"
-
-export default async function exit(_, response: NextApiResponse) {
-  response.clearPreviewData()
-  response.writeHead(307, { Location: "/" })
-  response.end()
-}

pages/api/preview.tsx

-import { DrupalPreview } from "next-drupal"
-
-export default DrupalPreview()