Move unappealing exercises out of core track
Move unappealing exercises out of core track

We've noticed that the exercises that are basically just
- solving a math problem
- implementing a computer science algorithm
- reimplementing a basic datastructure
- implementing methods typically found in the standard library

These are unappealing to a lot of people, and less interesting to mentor.

The following exercises have been moved out of core:
- sum-of-multiples
- accumulate
- perfect-numbers
- matrix

ricemery/xscala,exercism/xscala