fix: cancel after releaseLock throws INVALID_STATE error

[alanshaw]

While testing locally I was unable to fetch a file because the stream was being cancelled after releaseLock was called.

TypeError [ERR_INVALID_STATE]: Invalid state: The reader is not attached to a stream
    at new NodeError (node:internal/errors:393:5)
    at ReadableStreamDefaultReader.cancel (node:internal/webstreams/readablestream:837:28)
    at #processBatch (/Users/alan/Code/web3-storage/freeway/dist/worker.mjs:24428:16) {
  code: 'ERR_INVALID_STATE'
}

We don't need to releaseLock because we're the only reader so I've removed it and I no longer get this error.

[vasco-santos]

LGTM! This fix makes sense for the behaviour of the errors