drop rpm-ostree-base-db

[cgwalters]

I've been battling rpm-ostree-base-db and container layering. I did #5249 which helps, but it's still really not enough because ostree will end up re-hardlinking it all.

I think we need to just get rid of it. For the package layering case we should be able to just check out the rpmdb from the base commit? I don't know why we didn't do that originally.