core: Consolidate core and system state, remove system module & cleanups.

src/citra_qt/debugger/callstack.cpp

@@ -25,7 +25,7 @@ CallstackWidget::CallstackWidget(QWidget* parent) : QDockWidget(parent) {
 
 void CallstackWidget::OnDebugModeEntered() {
     // Stack pointer
-    const u32 sp = Core::g_app_core->GetReg(13);
+    const u32 sp = Core::AppCore().GetReg(13);
 
     Clear();