Delete all old contents after checking for important things to keep

[davidruegamer]

We should merge the folder lecture_DL_2017_2021 with the slides folder and not carry on all the old lecture material. For this we

• first need to check whether there is any relevant content that should go into the slides folder (even if it is not taught in classes)
• and then delete the lecture_DL_2017_2021 folder.

[AsjaFischer]

Last term we somehow got rid of the CNN architecture part. Especially the last slide (which actually corresponds to a view slides) is very helpful to put things together. I think we should keep it.