Add population_genotype data to variation endpoint

This adds the population genotype frequencies and counts for variation. I've removed the duplicate lines from t/test-genome-DBs/homo_sapiens/variation/genotype_code.txt too.
These things have a nasty habit of propagating. It's best if you nip it in the bud before someone else copies it again. Sadly I can't find the coding style guide any more, so you'll just have to take my word for it.