Debugger: further untangle of breakpoint item and marker

Change-Id: I9331912c1b53a0110479f46ef1e576676441ab75
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/breakhandler.h

@@ -93,7 +93,6 @@ private:
     friend class BreakHandler;
     friend class BreakpointManager;
     friend class BreakpointMarker;
-    friend class GlobalBreakpointMarker;
 
     void updateMarker();
     void updateMarkerIcon();