Revert "Add RapidSSL intermediate CA"

Reverts Shopify/active_utils#37.

@mutemule @EiNSTeiN- /cc @Shopify/payments.

Netregistry appear to have fixed up their chain (https://www.ssllabs.com/ssltest/analyze.html?d=paygate.ssllock.net), so I'd rather not keep this intermediate here explicitly.