remove redundant include guard and clarify macro usage

Signed-off-by: guptapratykshh <pratykshgupta9999@gmail.com>

Author: guptapratykshh

libs/core/type_support/include/hpx/type_support/is_replaceable.hpp

@@ -15,6 +15,11 @@
 
 namespace hpx::experimental {
 
+// P2786R13 defines a single feature-test macro __cpp_trivial_relocatability
+// that covers both the core-language keyword and the associated library traits
+// (std::is_trivially_relocatable, std::is_replaceable, std::relocate_at).
+// The language and library features are bundled in one proposal, so guarding on
+// this macro is sufficient to confirm that std::is_replaceable is available.
 #if defined(__cpp_trivial_relocatability)
     HPX_CXX_CORE_EXPORT template <typename T>
     struct is_replaceable : std::is_replaceable<T>

libs/core/type_support/include/hpx/type_support/is_trivially_relocatable.hpp

@@ -12,6 +12,12 @@
 
 namespace hpx::experimental {
 
+// P2786R13 defines a single feature-test macro __cpp_trivial_relocatability
+// that covers both the core-language keyword and the associated library traits
+// (std::is_trivially_relocatable, std::is_replaceable, std::relocate_at).
+// The language and library features are bundled in one proposal, so guarding on
+// this macro is sufficient to confirm that std::is_trivially_relocatable is
+// available.
 #if defined(__cpp_trivial_relocatability)
     template <typename T>
     struct is_trivially_relocatable : std::is_trivially_relocatable<T>

libs/core/type_support/include/hpx/type_support/relocate_at.hpp

@@ -15,10 +15,6 @@
 #include <cstring>
 #include <type_traits>
 
-#if defined(HPX_HAVE_P1144_STD_RELOCATE_AT) ||                                 \
-    defined(__cpp_trivial_relocatability)
-#include <memory>
-#endif
 
 namespace hpx::detail {