Rework which practice exercises should be listed in the concept tree

[sanderploegsma]

Currently the concept tree on the Java track is very hard to follow due to the large number of exercises listed for each concept. The reason this is happening is because all practice exercises list a number of concepts in their practices configuration. This probably started when migrating to Exercism v3.

The problem with this is that the concepts listed for each exercise only mean that that exercise is related to that concept. There is no way that there are 33 exercises to help a student practice the strings concept... Instead, every concept should contain a few hand-picked practice exercises that actually help a student to practice that concept. For the strings concept for example, some candidates are isogram and isbn-verifier.

So, the goal of this issue is to arrive at a concept tree that contains a relevant handful of practice exercises for each concept. A good example of this is the concept tree on the Python track.