Remove build scaffolding for OpenSSL from the "host" variant of the build

OpenSSL is only needed in the "main" variant of the build as of ("Split nts.h
into 2 parts so nts.h doesn't need openssl/ssh.h") so we don't need to check
for it in the "host" variant and we also don't need to list it as a build
dependency in several waf units.

This partially reverts ("Explicitly depend on libcrypto where we need it, for
now")

Fixes GitLab issue 573