Remove ZARR_V3_EXPERIMENTAL_API flag

[jhamman]

In #1032, the ZARR_V3_EXPERIMENTAL_API environment variable was added to the Zarr-Python code base to force users to opt in to the draft implementation. Once Zarr-Python is spec-compliant, we should remove this option.

[jhamman]

Closing as this is now done on the v3 branch. See #1583 for more context on the v3 effort.