[dotest] Remove multiprocessing

[dotest] Remove multiprocessing

Now that the Xcode project is removed, I want to focus on dotest as a
test framework, and remove its driver capabilities for which we already
rely on llvm's lit. Removing multiprocessing is the first step in that
direction.

Differential revision: https://reviews.llvm.org/D65311

llvm-svn: 367331