Remove useless _id.
Remove useless _id.

Putting _id in subdocuments is not recommended in MongoDB. In fact,
those guys were uniquely described by their name. We should use
$addToSet for those fields.

Widukind/dlstats,mmalter/dlstats,MichelJuillard/dlstats,MichelJuillard/dlstats,MichelJuillard/dlstats,mmalter/dlstats,Widukind/dlstats,mmalter/dlstats