Remove biosample.assay_type

[jonathancrabtree]

Remove all references to biosample.assay_type; it will be removed from the C2M2 specification and schema.

[jonathancrabtree]

It looks like cfde_fair:12 ("biosamples with assay") is the only metric affected by this change. As an aside, this metric was previously calculated by joining through file and examining file.assay_type instead of biosample.assay_type. So we could potentially revert to the original formulation, but that query was inconsistent with at least one other in the set (probably files with assay) and so I think it makes more sense to simply remove cfde_fair:12.

[jonathancrabtree]

Current metrics display (prior to rolling out the biosample.assay_type deletion). Note the 3rd metric in the left column, "biosamples with assay":