gdal tools: disconnect canvas signal after building vrt (fixes #15389)

(cherry picked from commit e963fff)

Author: jef-n

python/plugins/GdalTools/tools/doBuildVRT.py

@@ -78,6 +78,7 @@ def initialize(self):
     def onClosing(self):
         # disconnect from mapCanvas.layerChanged() signal
         self.disconnect(self.iface.mapCanvas(), SIGNAL("layersChanged()"), self.onVisibleLayersChanged)
+        self.disconnect(self.iface.mapCanvas(), SIGNAL("layersChanged()"), self.switchLayerMode)
         BasePluginWidget.onClosing(self)
 
     def onVisibleLayersChanged(self):