Don't set box-forced in Cartopy example.

[QuLogic]

It is deprecated in Matplotlib 2.2, removed in 3.1, and appears to have no effect on the result.

• Closes box-forced is removed in Matplotlib 3.1 #3219
• [N/A] Tests added
• Passes black . && mypy . && flake8
• [N/A] Fully documented, including whats-new.rst for all changes and api.rst for new API

[max-sixty]

Thanks @QuLogic !

I'll merge - if you want to add to whatsnew you could do that it #3274