eventually remove generate-yum-provenance

[Habbie]

When PowerDNS/pdns#14990 is merged, I suspect the only thing still depending on helpers/generate-yum-provenance.py is el-7 builds in pdns release branches, which we will hopefully never ever run again - and also, those are pinned to a current or older version of pdns-builder. Perhaps "eventually" in this issue title means "right now is fine"!