misplaced metadata creation methods on AsyncArray class

[d-v-b]

the AsyncArray class defines static methods for creating array metadata. I don't think there's any reason for these to be methods on AsyncArray, rather than plain functions.

[d-v-b]

see

zarr-python/src/zarr/core/array.py

Line 647 in ecc2d77

def _create_metadata_v2(

and

zarr-python/src/zarr/core/array.py

Line 552 in ecc2d77

def _create_metadata_v3(