Remove obsolete benchmark for inserting many child items

Remove obsolete benchmark for inserting many child items

I added this benchmark quite a long time ago in order to check that the
changes in dc6322dc090bcaec40d75522debad1edfb25b27a do not make
inserting many child items into the model slower. The test tries to
insert child items of different subfolders at the same time, which
cannot happen during normal usage because KDirLister only tells us about
one subfolder at a time.

The code was commented out a long time ago because it is no longer
compatible with the (greatly improved) new internal structure of
KFileItemModel. I think that it's not worth porting the benchmark
because it fulfilled its intended purpose a long time ago. Even if
someone decides to make a simplified working new benchmark out of it at
some point, it does not make much sense to keep this commented-out code
around for years. It can still be found in the git history - after all,
that's what version control systems are for.