citra-qt: Remove callstack widget

Appears to be currently broken, and given the complexity of doing this
for ARM code without debugging information, should probably be left to
an external tool or library. Use the GDB stub instead.

Closes #586

src/citra_qt/main.h

@@ -10,7 +10,6 @@
 #include <QTimer>
 #include "ui_main.h"
 
-class CallstackWidget;
 class Config;
 class EmuThread;
 class GameList;
@@ -152,7 +151,6 @@ private:
     ProfilerWidget* profilerWidget;
     MicroProfileDialog* microProfileDialog;
     RegistersWidget* registersWidget;
-    CallstackWidget* callstackWidget;
     GPUCommandStreamWidget* graphicsWidget;
     GPUCommandListWidget* graphicsCommandsWidget;
     GraphicsBreakPointsWidget* graphicsBreakpointsWidget;