Remove deprecated lecture visibleDate #11479
Description
Context
We decided to deprecate the visibleDate property of lectures as it has no practical use (lecture contents have their own release date anyways and there is no point in hiding if and when a specific lecture will take place - most of the time users know this anyways already). In #11436 we commented out related code as we wanted to monitor user feedback before removing the code completely. In this issue the deprecation should either be reverted or the related code should be removed completely.
TODOs
If we decide to remove the visibleDate, remove it from the entity, write a migration that drops the according column and remove code references of the properties getter and setter. Also take care of the commented out code from #11436 : Each commented out code piece is marked with a comment explaining the context, mentioning this issue and explaining what to do depending on whether the code should be removed entirely or changes should be reverted. Search for all occurences of "TODO: #11479" and follow the instruction (possibly also optimize the code for the new situation of visibleDate not existing anymore).