terminal: drop unfinished code

terminal: drop unfinished code

This drops the libsystemd-terminal and systemd-consoled code for various
reasons:

 * It's been sitting there unfinished for over a year now and won't get
   finished any time soon.

 * Since its initial creation, several parts need significant rework: The
   input handling should be replaced with the now commonly used libinput,
   the drm accessors should coordinate the handling of mode-object
   hotplugging (including split connectors) with other DRM users, and the
   internal library users should be converted to sd-device and friends.

 * There is still significant kernel work required before sd-console is
   really useful. This includes, but is not limited to, simpledrm and
   drmlog.

 * The authority daemon is needed before all this code can be used for
   real. And this will definitely take a lot more time to get done as
   no-one else is currently working on this, but me.

 * kdbus maintenance has taken up way more time than I thought and it has
   much higher priority. I don't see me spending much time on the
   terminal code in the near future.

If anyone intends to hack on this, please feel free to contact me. I'll
gladly help you out with any issues. Once kdbus and authorityd are
finished (whenever that will be..) I'll definitely pick this up again. But
until then, lets reduce compile times and maintenance efforts on this code
and drop it for now.