Revert "Extension: Mark the default constructor of chrono::duration as conditionally noexcept"; this breaks the gcc5 bot for C++11

This reverts commit c8ca15c95c4c0d6d1356500d5fe49a319ea4ca01.

llvm-svn: 372546

Author: mclow

libcxx/include/chrono

@@ -1063,7 +1063,7 @@ public:
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
 #ifndef _LIBCPP_CXX03_LANG
-        duration() noexcept(noexcept(_Rep())) = default;
+        duration() = default;
 #else
         duration() {}
 #endif