Do not echo back on successful POST on api/transaction

At the moment, after a transaction is successfully POSTed, we just echo it back [with the hash](https://github.com/bosnet/sebak/blob/aa4ff033fed5c18a2e36982b8b7f93c3e7553030/lib/node/runner/api/transaction_post.go#L50-L52).
This is not actually useful to anyone: the caller can just save the transaction, and since the tx is signed, [s]he already know the hash.

Start in lib/node/runner/api/transaction_post.go at the linked response-handling lines 50–52. Trace the successful POST path and confirm the intended behavior for a signed transaction; done means a successful POST no longer echoes the transaction and hash back to the caller.

Written by the indexing model from the issue text.