Fix processing modeler joins are drawn doubled

OCD fix: joins are actually being drawn with two lines going
in both directions. This is totally 100% unacceptable, so
this commit fixes it so they are correctly drawn only with one line.

python/plugins/processing/modeler/ModelerArrowItem.py

@@ -119,4 +119,6 @@ def paint(self, painter, option, widget=None):
 
         for point in self.endPoints:
             painter.drawEllipse(point.x(), point.y(), 6, 6)
-        painter.strokePath(self.shape(), painter.pen())
+
+        painter.setBrush(Qt.NoBrush)
+        painter.drawPath(self.path())