Remove crate.io support

crate.io has been discontinued. The crate.io import task is using an API that returns 404 and the existing links to crate.io project pages redirects to PyPI. This pull request removes all support for crate.io from RTD.

If you don't want to break the RTD API by removing the crate_url field from the project model, skip the last commit.

I guess this can be replaced by integration with the next generation PyPI called [Warehouse](https://github.com/pypa/warehouse) when its APIs are getting ready for use.