Removed sbt build.

Difficult though it may be to accept, it must go. We couldn't keep it
working with active maintenance; after eight months of neglect there is
no chance. Nobody is working on it or using it. The code will remain in
the history if anyone wants it.

One of the most annoying experiences one can have when building a new
project is finding out one has been fiddling with an abandoned build
system which isn't even expected to work. Sometimes I check out a scala
project and there is a build.xml, a pom.xml, and a project directory.
We should not be among those who sow such build confusion.