Remove asciitree dependency

[normanrz]

Remove asciitree dependency. I couldn't find any use of it in the codebase.

Fixes #1370
Contributes to #2391

[d-v-b]

thanks for cleaning this up!

[normanrz]

cc @oanegros

[jakirkham]

Would we want to get this one too?

zarr-python/docs/roadmap.rst

Lines 570 to 579 in 3dd04ce

	Today, Zarr-Python has the following required dependencies:
	.. code:: python
	dependencies = [
	'asciitree',
	'numpy>=1.20,!=1.21.0',
	'fasteners',
	'numcodecs>=0.10.0',
	]

It's possible the answer is no. Just wanted to ask for completeness

[d-v-b]

I think that statement is scoped to the time when it was written, which was a time when those were in fact the minimum dependencies. So removing asciitree would actually make the document inaccurate.