MemoryUtil: Remove unimplemented/unused functions

GuardMemoryMake/GuardMemoryUnmake are unimplemented prototypes.
GetPageSize is an unused function.

Source/Core/Common/MemoryUtil.h

@@ -21,12 +21,4 @@ void UnWriteProtectMemory(void* ptr, size_t size, bool allowExecute = false);
 std::string MemUsage();
 size_t MemPhysical();
 
-void GuardMemoryMake(void* ptr, size_t size);
-void GuardMemoryUnmake(void* ptr, size_t size);
-
-inline int GetPageSize()
-{
-  return 4096;
-}
-
 }  // namespace Common