crate.io: Remove import task

crate.io has been discontinued and the API no longer responds.
crate.io: Remove from project details page

As the import task fails, the data may be stale and the links only redirect to
PyPI.
crate.io: Don't use crate.io as index for pip

crate.io is discontinued, and PyPI got a nice CDN some time ago, making it
fast and reliable.
crate.io: Remove the crate_url field
Merge pull request #615 from jodal/remove-crate-support

Remove crate.io support