device_memory: Remove unused system member

This isn't used by anything in particular, so it can be removed.

src/core/device_memory.cpp

@@ -2,14 +2,11 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
-#include "core/core.h"
 #include "core/device_memory.h"
-#include "core/memory.h"
 
 namespace Core {
 
-DeviceMemory::DeviceMemory(System& system) : buffer{DramMemoryMap::Size}, system{system} {}
-
+DeviceMemory::DeviceMemory() : buffer{DramMemoryMap::Size} {}
 DeviceMemory::~DeviceMemory() = default;
 
 } // namespace Core