Remove the OpenPGP support
Currently we provide limited support for establishing connections with OpenPGP certificates and keys. It is limited because it has no specific use-cases that it addresses, and mostly serves as a technology preview. As the OpenPGP certificate for HTTPS (or TLS in general) never got any traction and GnuTLS is the only implementation supporting it, we should drop that code completely when an opportunity is given.
Deprecation has already occurred in #102 (closed).