Made cube example not use glActiveTexture.

We're setting the default texture unit anyway, and glActiveTexture would
require resolving through QOpenGLFunctions.

Task-number: QTBUG-24555
Change-Id: Id8d660baaa1532e7b8e623673f501703c76fac65
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

examples/opengl/cube/mainwidget.cpp

@@ -173,8 +173,7 @@ void MainWidget::initShaders()
 //! [4]
 void MainWidget::initTextures()
 {
-    // Loading cube.png to texture unit 0
-    glActiveTexture(GL_TEXTURE0);
+    // Loading cube.png
     glEnable(GL_TEXTURE_2D);
     texture = bindTexture(QImage(":/cube.png"));