Remove the Rio magnetic data

[leouieda]

Now that we have the Great Britain magnetic survey (which has a proper
open license) we don't need the Rio one (which is ambiguous). Remove the
dataset and loading function. Replace its use in the gallery with the
Britain data.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.

[santisoler]

Seems good to me @leouieda! 🥇