getitems follow up work

[jakirkham]

PR ( #1131 ) added getitems

In looking over the code we noticed some things that could benefit from cleanup

This check shouldn't be needed:

zarr-python/zarr/core.py

Line 1995 in b3cafa9

and hasattr(self.chunk_store, "getitems")

Also this PR ( #1040 ) might be worth reviewing to see if any pieces from it can be included

[jakirkham]

cc @madsbk (for vis)