[bug] remove loseConnection on decryption error during sync

When there's an error parsing a document from the incoming stream during sync, the doc stream receiver will log the error and attempt to shutdown the connection. Despite that, the transport held by the fetcher is a wrapper proxy for the real transport that implements the producer/consumer pattern. Because of that, calling loseConnection directly will raise the following exception:

exceptions.AttributeError: 'TransportProxyProducer' object has no attribute 'loseConnection'

This commit removes that call, and replaces it for nothing as the producer will already have been stopped in case of an error.