Revert "QQuickItem: Fix effective visibility for items without parent"

This breaks applications that use QQmlPropertyList to store QQuickItem
and don't set a parentItem for them.

Ref: musescore/MuseScore#17276

This reverts commit 45c22a0221937682f4496801a495458a00f76d3a.

src/quick/items/qquickitem.cpp

@@ -6081,8 +6081,10 @@ void QQuickItem::setEnabled(bool e)
 
 bool QQuickItemPrivate::calcEffectiveVisible() const
 {
-    // An item is visible if it is a child of a visible parent, and not explicitly hidden.
-    return explicitVisible && parentItem && QQuickItemPrivate::get(parentItem)->effectiveVisible;
+    // XXX todo - Should the effective visible of an element with no parent just be the current
+    // effective visible?  This would prevent pointless re-processing in the case of an element
+    // moving to/from a no-parent situation, but it is different from what graphics view does.
+    return explicitVisible && (!parentItem || QQuickItemPrivate::get(parentItem)->effectiveVisible);
 }
 
 bool QQuickItemPrivate::setEffectiveVisibleRecur(bool newEffectiveVisible)

tests/auto/quick/qquickitem/tst_qquickitem.cpp

@@ -989,9 +989,7 @@ void tst_qquickitem::setParentItem()
 
 void tst_qquickitem::visible()
 {
-    QQuickWindow window;
     QQuickItem *root = new QQuickItem;
-    root->setParentItem(window.contentItem());
 
     QQuickItem *child1 = new QQuickItem;
     child1->setParentItem(root);