In GFF3 and GTF headers, the assembly accession was being prefixed by the value of the meta_key 'assembly.web_accession_source'. This is incorrect - this value is intended to be used by the webcode, in order to decide whether to create hyperlinks to ENA or NCBI, and thus the meta_value is 'ENA' or 'NCBI'. We don't want to prefix the accession with either of those values - it is an INSDC identifier, and it's misleading/confusing to associate it with a specific member.