Remove LegacyJSON codec

[jrbourbeau]

Fixes #225

TODO:

• Unit tests and/or doctests in docstrings
• tox -e py38 passes locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• tox -e docs passes locally
• AppVeyor and Travis CI passes
• Test coverage to 100% (Coveralls passes)

[pep8speaks]

Hello @jrbourbeau! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file numcodecs/__init__.py:

Line 102:1: E402 module level import not at top of file

Comment last updated at 2020-04-02 18:41:06 UTC

[jakirkham]

Thanks James! 😄