Drop old CHANGELOG file linking to GitHub releases
Drop old CHANGELOG file linking to GitHub releases

As we now use rever to cut releases and that makes a `CHANGELOG.rst` in
the top-level directory, there is no need to point users to GitHub
releases using this `CHANGELOG.md` as well. So go ahead and drop it for
simplicity.

ocefpaf/conda-smithy,ocefpaf/conda-smithy,conda-forge/conda-smithy,conda-forge/conda-smithy