Avoid preceding select when deleting blob links

Avoid preceding select when deleting blob links.

Additionally, this MR uses a digest.Digest as key for blob related methods whenever possible, instead of requiring a models.Blob struct. The reasoning is that with the partitioned schema, the blobs table no longer has an id, and the primary key now is the digest. Therefore, we can unequivocally refer to blobs by their digest. Furthermore, when calling these methods we always have the blob digest, but we may not have a models.Blob struct, and building one with just the digest to pass it as an argument to these functions is less than ideal.

Related to #206 (closed)