Remove codecov

[jakirkham]

Fixes #1390

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

Merging #1391 (c2385ef) into main (4b0705c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #1391   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           36        37    +1     
  Lines        14799     14855   +56     
=========================================
+ Hits         14799     14855   +56     

Impacted Files	Coverage Δ
zarr/_storage/store.py	100.00% <100.00%> (ø)
zarr/context.py	100.00% <100.00%> (ø)
zarr/core.py	100.00% <100.00%> (ø)
zarr/storage.py	100.00% <100.00%> (ø)
zarr/tests/test_storage.py	100.00% <100.00%> (ø)
zarr/tests/test_storage_v3.py	100.00% <100.00%> (ø)
zarr/tests/test_util.py	100.00% <100.00%> (ø)
zarr/tests/util.py	100.00% <100.00%> (ø)
zarr/util.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[jakirkham]

@MSanKeys963 looks like this works 🎉 Going to go ahead and merge to fix CI

[MSanKeys963]

Thanks, @jakirkham! 🙏🏻

Also, sent a PR for adding this in the upcoming release notes, #1392.