remove deprecated stuff, add asserts

Using `discid.DiscId()` is now deprecated for half a year and was already deprecated when 1.0 was reached. `discid.DEFAULT_DEVICE` was already removed in 1.0.

So we should probably remove support for `discid.DiscId()` altogether.

With that step we can drop the now internal *test_emptyness* test.
We might place some `assert(self._success)` to fail early with broken code.
This is in reference to #37.