Remove double index creation from async DataTree open

The zarr v3 async backend was creating indexes inside
_open_datatree_from_stores_async, but the framework layer
(_datatree_from_backend_datatree in api.py) already handles
index creation for all backends. This caused indexes to be
created twice per node and made create_default_indexes=False
silently ignored.

Remove the index creation from the backend to restore the
clean main architecture: backends open datasets without
indexes, the framework creates them once.

Author: aladinor

xarray/backends/zarr.py

@@ -2009,8 +2009,6 @@ async def _open_datatree_from_stores_async(
         max_concurrency: int | None = None,
     ) -> DataTree:
         """Open datatree groups concurrently using a dedicated executor."""
-        from xarray.backends.api import _maybe_create_default_indexes_async
-
         if max_concurrency is None:
             max_concurrency = 10
 
@@ -2055,7 +2053,6 @@ def _sync_open():
                     )
 
             ds = await loop.run_in_executor(executor, _sync_open)
-            ds = await _maybe_create_default_indexes_async(ds, executor=executor)
             if group:
                 group_name = str(NodePath(path_group).relative_to(parent))
             else: