calamari_ocr dependency outdated and heavy

We still have `calamari_ocr == 0.3.5` in `install_requires`. That's quite a drag IMO: it is quite out-dated, pulls in Tensorflow/Keras (themselves outdated), and probably not really necessary (since we've had ocrd_calamari for quite a while now).

Can we remove this?

(I see references to Calamari in `div.eval`, `div.auswerter.runcalamari` and `aio.runcalamari`.)