Remove a dead #ifdef.

Unix/Threading.inc should never be included on _WIN32. See also
https://reviews.llvm.org/D30526#1082292


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331151 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed