Chart Server dependency

[rainwoodman]

Google chart server has been deprecated in 2012, and the notification for turning it down was in 2019:

https://groups.google.com/g/google-chart-api/c/rZtHTyYgyXI

The servers can be gone at any moment at this point.

These cartopy tests still seem to rely on the chart server API:

https://github.com/SciTools/cartopy/blob/da6a8c521f614abea4d16e659b3c87ec80025a66/lib/cartopy/tests/test_img_tiles.py#L39

Could we remove the chartserver dependencies? What is the best path forward? How deep does cartopy depend on the chart server API?

[QuLogic]

It is, AFAICT, only used for testing in that file.