crypto: remove use of this._readableState

Per #445 this removes a reference to this._readableState in hash._flush. It was
used to get the encoding on the readable side to pass to the writable side but
omitting it just causes the stream to handle the encoding issues.

PR-URL: https://github.com/iojs/io.js/pull/610


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>