[libcxx] remove unused code

The macro _LIBCPP_UNROLL_LOOPS isn't used anywhere
so the code was dead.

Differential Revision: https://reviews.llvm.org/D26991

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@288143 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed