Don't refresh the entire window when we render the plotter

This means the window is only updated when the plotter changes, rather
than effectively every possible frame.

Reviewed by: Marco Martin

CCBUG: 348385

src/qmlcontrols/kquickcontrolsaddons/plotter.cpp

@@ -709,9 +709,6 @@ void Plotter::render()
 
     // Delete the VBO
     glDeleteBuffers(1, &vbo);
-    if (window()) {
-        window()->update();
-    }
 }
 
 QSGNode *Plotter::updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData *updatePaintNodeData)