Remove explicit checking of existence of retrieved folders in the parsers

[sphuber]

As of aiida-core==1.4.0, the engine itself will check for the presence of the retrieved folder. If it is missing, which is really just a bug in aiida-core the engine will terminate the process and not call the parser. Anyway there would be no point since there is nothing to parse. This means that now if the parser is called, it is guaranteed that the retrieve folder exists and so we no longer have to explicitly check for it.