remove mnist example

[haesleinhuepf]

Hi Johannes @jo-mueller ,

do you still need the MNIST example notebook? https://github.com/BiAPoL/Quantitative_Bio_Image_Analysis_with_Python_2022/blob/main/docs/day2e_deep_learning_segmentation/02_MNIST_in_torch.ipynb

I propose removing it as it has nothing to do with bio-image analysis and the time is tight in the deep-learning session.

Thanks! Robert