remove MetricSpace and BKTree

Let's leave numerics to spire. They have `MetricSpace` [too](https://github.com/non/spire/blob/2.10.0/core/src/main/scala/spire/algebra/MetricSpace.scala), and if somebody's interested in `BKTree`, it could be resurrected in [spire-contrib](https://github.com/typelevel/spire-contrib).