Remove the last two videos in 4.2

[Austing767]

The last two videos are private and unavailable, respectively.

Could maybe try and find replacements on similar topics, but I don't know what the videos were originally.

[Austing767]

After further discussion, we do need four videos because of the point the text is trying to make about not running in parallel but switching between tasks so fast. It could maybe be done with two videos, but I'll just add two videos about linked lists.