[CodeGen] Remove an obsolete macro test (NFC)

We can drop the macro test because we require C++17 and GCC 7.4 these
days.  Note that std::is_trivially_copyable_v is a C++17 feature.

Author: kazutakahirata

llvm/unittests/CodeGen/TypeTraitsTest.cpp

@@ -16,13 +16,10 @@
 
 using namespace llvm;
 
-#if __has_feature(is_trivially_copyable) || (defined(__GNUC__) && __GNUC__ >= 5)
 static_assert(std::is_trivially_copyable_v<PressureChange>,
               "trivially copyable");
 static_assert(std::is_trivially_copyable_v<SDep>, "trivially copyable");
 static_assert(std::is_trivially_copyable_v<SDValue>, "trivially copyable");
 static_assert(std::is_trivially_copyable_v<SlotIndex>, "trivially copyable");
 static_assert(std::is_trivially_copyable_v<IdentifyingPassPtr>,
               "trivially copyable");
-#endif
-