export_v2: Allow paper_urls that end with pubmed
export_v2: Allow paper_urls that end with pubmed

There had been an upstream issue in fauna resulting in paper_url in the form of https://www.ncbi.nlm.nih.gov/pubmed. This has been resolved and this check can be removed.
nextstrain/augur,nextstrain/augur,blab/nextstrain-augur,nextstrain/augur