Remove --stats if/when we upgrade to Snakemake v8

[joverlee521]

Follow up to #451.

We are using the Snakemake --stats flag to get workflow stats for GISAID and GenBank ingest to help with #446. If/when we ever upgrade Snakemake to v8, the --stats flag is no longer supported and will need to be removed.

Upgrading Snakemake is dependent on updating the nextstrain docker-base image and then running the update-image workflow in this repo since the workflows are using the nextstrain/ncov-ingest Docker image.

Alternatively, we can pin Snakemake <8 in this repo, but we can make that decision later.