Remove non-existent signal connection from IncrementalPlot.

I don't think it's needed anywhere in QGC since this has been a bug for a long time.

src/ui/linechart/IncrementalPlot.cc

@@ -100,8 +100,6 @@ IncrementalPlot::IncrementalPlot(QWidget *parent):
     resetScaling();
 
     legend = NULL;
-
-    connect(this, SIGNAL(legendChecked(QwtPlotItem*,bool)), this, SLOT(handleLegendClick(QwtPlotItem*,bool)));
 }
 
 IncrementalPlot::~IncrementalPlot()