Debugger: Delete breakpoint markers on session switch

There were some left-over "ghost markers" when switching sessions,
which should be gone now.

This also pointed to a wrong use of SessionManager::aboutToUnloadSession
which mustn't save data re-created in a save/load cycle as it
is called during any session load, including the very first on startup
no such data (e.g. breakpoints) have been created yet.

Change-Id: I237a9e3500a786841618941ce85a97f7bd4f318c
Fixes: QTCREATORBUG-22856
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/debugger/breakhandler.h

@@ -312,7 +312,6 @@ private:
 
     void loadSessionData();
     void saveSessionData();
-    void aboutToUnloadSession();
 
     bool contextMenuEvent(const Utils::ItemViewEvent &ev);
     void gotoLocation(const GlobalBreakpoint &gbp) const;