Remove curl-ca-bundle

This is no longer used, directly or indirectly, by anything in core. I want to discourage its use, since providing an unsigned bundle of certificates and letting users place their trust in it is a poor practice.

The openssl formula provides a cert file that is bootstrapped using certificates from the system keychain. Additional certificates can be added in `$(brew --prefix)/etc/openssl/certs`, where they will be picked up by openssl. This is a far more reasonable solution.