Switch to OpenSSL

To follow the Apertis guidelines, which chooses OpenSSL as TLS stack, switch
systemd to be build against it.

In the same line drop systemd-journal-remote and its dependency
libmicrohttpd-dev since it can only be built against GnuTLS but has no value
for Apertis.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>