fix(ext/node): remove fs.promises.fstat, not a public Node.js API (#32289)
Closes https://github.com/denoland/deno/issues/27423
`fs.promises` in Node.js does not expose `fstat` or other `f`-prefixed
APIs. The promise-based equivalent is `FileHandle.stat()`, which is
already supported via `handle.ts`.
This removes the erroneous `fs.promises.fstat` export from both
`polyfills/fs/promises.ts` and the `promises` object in
`polyfills/fs.ts`.