Remove AB3 tutorials

There are AB3 tutorials in the codebase under the 'tutorials' folder.

They need to be removed because they have been relocated to an online
guide.

Let's remove them and the 'tutorials' folder from this codebase.

This change avoids redundancy and keeps the codebase clean.

Refer to the new location of the tutorials: https://se-education.org/guides/tutorials/ab3.html.