Move from gnutls + openssl to only openssl
Previously, all our recipes were linking to either openssl or gnutls. As a result, we were shipping two TLS implementations, which is messy because it means consumers of Cerbero and our binaries had to configure the certdb twice (thrice if you count glib-networking), do a security audit of two TLS implementations, and generally had increased binary sizes because of this.
Since we require openssl for the DTLS plugin (used by webrtcbin), and all the other recipes can pick either openssl or gnutls, we have to go with openssl.
This involved importing patches from vcpkg to port librtmp to the latest OpenSSL 1.1.1 version since upstream is non-existent.
We have also deleted the following recipes: gmp, nettle, libtasn1, gnutls.
This is the simpler alternative to !368 (closed).