dbSNP: remove rs_ids INFO after ID annotation
dbSNP: remove rs_ids INFO after ID annotation

Avoids complicated merging downstream when different regions may
or may not have rs_ids INFO tags (and thus an INFO line in the VCF
header). Thanks to @vladsaveliev

chapmanb/bcbio-nextgen,lbeltrame/bcbio-nextgen,vladsaveliev/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen,chapmanb/bcbio-nextgen,a113n/bcbio-nextgen,vladsaveliev/bcbio-nextgen,biocyberman/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,biocyberman/bcbio-nextgen,lbeltrame/bcbio-nextgen,biocyberman/bcbio-nextgen