Remove CERTIFICATES

[shikhar]

As of hyperium/tonic#660, it is convenient to simply enable the tonic tls-webpki-roots crate feature.

With that feature enabled, one can omit tls_config() on the Channel and everything JustWorks:tm:

[mechiru]

@shikhar Thank you for your suggestions.
It cannot be deleted right now. That's because the user must explicitly enable the tls-webpki-roots feature.
(Some people don't want to rely on webpki-roots crate.)
So I would like to consider how to use feature gate for this crate as well. This method allows the user to decide which to use.
How about that?

[shikhar]

Not a big deal, users can just choose to not use the export. But I wonder in what situation a user would be ok with the certs exported by this crate but not webpki-roots.