Remove the verde.datasets module

[leouieda]

This module has been deprecated in #277. The CheckerBoard class will be the only one there and even it is getting moved to a different module (#349). In Verde 2.0.0 #331 we should remove the entire module.

Doing so will require using Ensaio to source our datasets for the docs instead of verde.datasets first.

[leouieda]

Once this is done, Pooch can be dropped as a dependency.