Data purge does not remove orphan SNPs

[ljwh2]

A user reported that they could find SNPs in the V2 REST API snp endpoint, that did not have associations.

Examples: 'rs2844702', 'rs3129179', 'rs3135406', 'rs2894083', 'rs2532921', 'rs6906873', 'rs2517449', 'rs9257618', 'rs2532920', 'rs3129096

These seem to be orphan records in the db (based on the first example), likely SNPs that were added and then later deleted by a curator. Since these Snps don't have any linked studies Data Purge can't remove them.

These only exist where a study is imported to curation db and SNPs are deltede later (this was the process for L1/L2 curation before we had curation statuses in depo-curation). It's possible we could just remove them manually, as the likelihood of generating more is much lower now we have changed the process.

Needs discussion.