Remove unused method Job::AddCommand.

I was going to update the comment referring to PipedJob, which was removed
some time ago, but then it turned out that this method is not actually used
at all.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190171 91177308-0d34-0410-b5e6-96231b3b80d8