ssl: Require SSL support in Qt

ssl: Require SSL support in Qt

There is no sane reason for not using SSL in the year 2020, plus
every distro should be shipping the Qt libraries with SSL support
enabled. Thus, make it mandatory for Quassel, too.

Going forward, this allows us to remove lots of fallback code paths
that make the code much more complex in many places, and that have
not been tested (or even been built) in a long time.