Remove non-existent lecture slide links

[zezl7]

Description

The hyperlinks to the lecture slides containers_demo.md and containers_slides.md are not valid/ not found anymore, seeing that the corresponding files are not inside the 02_virtualization_and_containers repository.

Checklist

• I made sure that the markdown files are formatted properly.
• I made sure that all hyperlinks are working. - indeed :)

• I added the new content to the README.md inside the topics subdirectory, e.g., 01_version_control/README.md.
• I added the new removed content to from the timetable.md in the root of this repository.

Note

Perhaps the timetable is currently being modified by the maintainers, which leads to the invalid links. In that case, this PR is not relevant, as changes are underway.

[zezl7]

Just noticed: virtualmachines_slides.md is also invalid.