[libc++] Remove workaround for broken __is_trivially_copyable on old GCC

All supported versions of GCC now do the right thing.

Differential Revision: https://reviews.llvm.org/D108997

Author: ldionne

libcxx/include/type_traits

@@ -3708,13 +3708,7 @@ _LIBCPP_INLINE_VAR _LIBCPP_CONSTEXPR bool is_standard_layout_v
 // is_trivially_copyable;
 
 template <class _Tp> struct _LIBCPP_TEMPLATE_VIS is_trivially_copyable
-#if __has_feature(is_trivially_copyable)
     : public integral_constant<bool, __is_trivially_copyable(_Tp)>
-#elif _GNUC_VER >= 501
-    : public integral_constant<bool, !is_volatile<_Tp>::value && __is_trivially_copyable(_Tp)>
-#else
-    : integral_constant<bool, is_scalar<typename remove_all_extents<_Tp>::type>::value>
-#endif
     {};
 
 #if _LIBCPP_STD_VER > 14 && !defined(_LIBCPP_HAS_NO_VARIABLE_TEMPLATES)

libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/is_trivially_copyable.pass.cpp

@@ -10,10 +10,6 @@
 
 // is_trivially_copyable
 
-// GCC has not implemented Core 2094 which makes volatile
-// qualified types trivially copyable.
-// XFAIL: gcc
-
 #include <type_traits>
 #include <cassert>
 #include "test_macros.h"