Since D6242 there is no more OpenSSL needed outside of the BIP70
feature. This diff cleans up the build system to reflect this.
Inspired by
https://github.com/bitcoin/bitcoin/pull/17165/commits/fcee10c2d028cba11416d902f5abf13fea7a65f4
and https://github.com/bitcoin/bitcoin/pull/17265/commits/8983ee3e6dd8ab658bd2caf97c326cc53ea50818,
but obviously different because we maintain BIP70.
Depends on D6251.