• No need to have both Google's and Apple's stock prices, one is enough.
• Replace ada.png (used once only) and grace_hopper.png by the 10x
  smaller grace_hopper.jpg.
• Replace ct.raw.gz (CT scan example) by s1045.ima.gz (MRI example,
  smaller).
• Remove unused None_vs_nearest-pdf.png (see also d35fc38), demodata.csv
  (there's still msft.csv as example csv file).

Overall this make the sample_data directory ~4x smaller (down to 500K,
which is smaller than nearly all the extension modules); we can perhaps
now consider always shipping sample_data with Matplotlib instead of
having its presence controlled by a flag.

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and pydocstyle<4 and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).