Remove singularity hub integration

[asmacdo]

It's gone, so we should remove that code. Hopefully we replace these usages with ReproNim/containers?

[yarikoptic]

well, we still provide "shim" for shub:// so old packages should still be accessible, but indeed there would be nothing "new" there. The point is -- if we used it for testing of singularity -- we could keep, if not -- indeed should get rid of. So what actual integration you have in mind?