BLD: Remove `options.package_data` from setup.cfg

[NickCrews]

Hello! Not sure if this is actually what we want, but I think this can simplify things.

Currently, we include data files in builds in 3 ways:

1. Listing them in MANIFEST.in
2. Listing them under options.package_data in setup.cfg
3. Setting include_package_data = True in setup.cfg

I believe that it is possible (and therefore desirable, as it is more DRY and less spread out) to migrate everything in number 2 to number 1, and still have the same behavior.

This is per both https://setuptools.pypa.io/en/latest/userguide/datafiles.html and from experiments in https://github.com/abravalheri/experiment-setuptools-package-data, where it is shown that if you have include_package_data = True, then MANIFEST.in is sufficient, you don't need options.package_data, both for sdists and wheels.

It looks like include_package_data = True quietly snuck during #38852. Before this, I think the package_data was needed, but now it is not.

A small local test I think showed this to be true, but I might have messed it up so not sure. Probably artifacts from CI would tell the story.

@fangchenli I think you might be the one to look at this? Thank you! See the attached PR