`Store.delete` has inconsistent implementations

[d-v-b]

LocalStore.delete removes entire directory trees, i.e. many objects:

zarr-python/src/zarr/storage/local.py

Lines 206 to 213 in 4c3081c

	async def delete(self, key: str) -> None:
	# docstring inherited
	self._check_writable()
	path = self.root / key
	if path.is_dir(): # TODO: support deleting directories? shutil.rmtree?
	shutil.rmtree(path)
	else:
	await asyncio.to_thread(path.unlink, True) # Q: we may want to raise if path is missing

MemoryStore.delete removes single keys, i.e. just one object:

zarr-python/src/zarr/storage/memory.py

Lines 134 to 140 in 4c3081c

	async def delete(self, key: str) -> None:
	# docstring inherited
	self._check_writable()
	try:
	del self._store_dict[key]
	except KeyError:
	pass

We should pick one of the two options. The fact that we got this far with such skew also means that our store tests should be made more extensive.

[jhamman]

Good catch! Store.delete should be for a single key. In #2430, I introduce delete_dir for removing all objects below a node in a hierarchy.