Remove the 3rd line stuff from unassigned fq entries

[stu2]

This gives properly formatted "unassigned" fastq files

[serine]

@stu2 thanks for contributing. Just to clarify third line in the fastq file messing with you workflows? Like you specifically need it to be + or this patch is more generic, like third line in the fastq file is pretty much unused and therefore shouldn't haven't any text?

thanks

[stu2]

Yes, I am not sure whether it is officially allowed for fastq format, but having 3rd line with anything other than a '+' messes with some downstream tools.