Massive performance loss of QTreeView::expandAll

Moving from Qt 6.2.4 to Qt 6.6 on macOS showed a huge performance degradation with using `QTreeView::expandAll()` on a tree view with approx. 92k items.

Following discussion on the forum identified this change as the cause for the degradation.

Removal of `resetModelIfNeeded` restores the performance of `expandAll()` from *47 seconds* back down to below a second.

Attached is the forum's testcase which creates a treeview with 92k items. Clicking on expand button takes 40+ seconds on macOS. Detailed timings are provided in the forum post as well as screenshots tracing Qt with Apple Instruments.