Remove build-conflicts on openssl, but explicitly use the fallback

Conflicting with openssl causes too much trouble, since it also transitively conflicts with devscripts and ca-certificates. Also, if curl or wget happen to be installed, dh params will be downloaded from a remote host, which may be dangerous if it’s taken over by a hostile entity.

Instead, remove the build-conflicts and comment out the generation and the download.

Debian bug: #987397