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.