Two small optimizations in KFileItemModel::removeItems()

Two small optimizations in KFileItemModel::removeItems()

1. It seems that it really can happen that KFileItems that we get from
   the dir lister's itemsDeleted signal are not in the model any more,
   e.g., if a folder where hidden files are shown is left and a folder
   where hidden files are not shown is entered. There is no need to
   output warnings then.
2. Remove the emptiness-check for the KFileItemList at the beginning.
   Even in the unlikely event that we do get an empty list, we return
   just a few lines later in the code.