[cleanup][cmake] Remove install directives

Merged Timo Koch requested to merge feature/remove-faulty-cmake-install-directives into master

This removes all install directives in lecture. They were faulty and/or incomplete. I currently don't see the point why lecture needs to be installed as the whole point is to play around with the application code. It's not a library. If the install features is needed for something please add them back in using a Python script so that they are easy to maintain.

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