Remove vendored zarr code

[TomNicholas]

We are apparently still vendoring some code from zarr-python, even though a while ago we added an explicit dependency on it (in #482). That vendored code should have been removed then in favour of directly importing things.

[TomNicholas]

This came up when I got this error trying to run tests locally

ImportError while loading conftest '/Users/tom/Documents/Work/Code/VirtualiZarr/conftest.py'.
conftest.py:16: in <module>
    from virtualizarr.manifests import ChunkManifest, ManifestArray
virtualizarr/__init__.py:3: in <module>
    from virtualizarr.accessor import (
virtualizarr/accessor.py:7: in <module>
    from virtualizarr.manifests import ManifestArray
virtualizarr/manifests/__init__.py:7: in <module>
    from virtualizarr.manifests.store import ManifestStore, ObjectStoreRegistry  # type: ignore # noqa
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
virtualizarr/manifests/store.py:20: in <module>
    from virtualizarr.vendor.zarr.core.metadata import dict_to_buffer
virtualizarr/vendor/zarr/core/metadata.py:6: in <module>
    from zarr.core.metadata.v3 import V3JsonEncoder
E   ImportError: cannot import name 'V3JsonEncoder' from 'zarr.core.metadata.v3' (/Users/tom/Documents/Work/Code/VirtualiZarr/.venv/lib/python3.13/site-packages/zarr/core/metadata/v3.py)

But actually this might be related to the zarr v3 dtypes refactor?

[maxrjones]

💯 agree on removing the vendored metadata code.

I think @norlandrhagen vendored https://github.com/zarr-developers/VirtualiZarr/blob/develop/virtualizarr/vendor/zarr/core/common.py to use a private function which we cannot rely on directly. So it might need to stay unless we find a better solution for https://discourse.pangeo.io/t/async-and-sync-interfaces/5190.