community/libgdata: disable unused demos

The demos are currently not packaged. One of them, demos/calendar/calendar-cli.c, also had an incompatible pointer types error with time_t on 32-bit arches, which led to rebuild failure there (found on the 3.21 x86 builder).

While there is a patch proposed upstream to fix the demo, it is probably simpler to disable them since they are not being used anyway.