Drop `ascii_kwargs` from `TreeHierarchy` constructor
Drop `ascii_kwargs` from `TreeHierarchy` constructor

We don't currently provide a way to set `ascii_kwargs` from `tree`. So
maybe we don't need the argument in `TreeHierarchy`'s constructor.
Besides it is trivial to set the `ascii_kwargs` after the fact.

zarr-developers/zarr-python,alimanfoo/zarr