[arc] Remove unittesting from arcconfig

Summary:
Unittests aren't working, and I don't think they ever were. Just remove
them, so that we don't have  to write `arc --nounit` all the time.

Reviewers: grosser, Meinersbur, bollu

Subscribers: bollu, pollydev, llvm-commits

Differential Revision: https://reviews.llvm.org/D46869