Revert commit r347904 because it broke older compilers

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@347908 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/language.support/support.limits/limits/numeric.limits.members/digits.pass.cpp

@@ -14,8 +14,6 @@
 #include <limits>
 #include <cfloat>
 
-#include "test_macros.h"
-
 template <class T, int expected>
 void
 test()
@@ -33,9 +31,6 @@ int main()
     test<signed char, 7>();
     test<unsigned char, 8>();
     test<wchar_t, std::numeric_limits<wchar_t>::is_signed ? sizeof(wchar_t)*8-1 : sizeof(wchar_t)*8>();
-#if TEST_STD_VER > 17
-    test<char8_t, 8>();
-#endif
 #ifndef _LIBCPP_HAS_NO_UNICODE_CHARS
     test<char16_t, 16>();
     test<char32_t, 32>();