irmin-pack: remove the forbid_empty_dir_persistence option

[samoht]

This should not be exposed as a configuration option - this doesn't exist in other data-model (and I'm not sure how users could make an informed choice here).

Instead, we should probably special-case the empty root tree in the backend, to make sure it is never written to disk (as this might be generate a collision issue with irmin-pack default configuration).