Removes {i2o,o2i}_SCT_signature from the CT public API

They may return if an SCT_signature struct is added in the future that
allows them to be refactored to conform to the i2d/d2i function signature
conventions.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>