[test] Remove workaround for C1XX empty parameter pack bug

Was VSO#109062. This bug was filed *4 years ago*. I submitted a workaround to enable the scoped_allocator_adaptor tests to pass. Bug fixed a week and a half later. This was either a waste of my time, or I've discovered that libc++ has magical bugfix-inducing powers. My money's on the latter.

llvm-svn: 304730