Simplify the KFileItemModel benchmark

The benchmark inserted KFileItemLists of different sizes in the model.
The intention was to verify that an O(N^2) complexity issue was fixed.
However, now that this original problem does not exist any more, the
benchmark results are much easier to read, and regressions are much
easier to spot at first sight, if only a single list size is used.

src/tests/kfileitemmodelbenchmark.cpp

@@ -81,8 +81,7 @@ void KFileItemModelBenchmark::insertAndRemoveManyItems_data()
     QTest::addColumn<KItemRangeList>("expectedItemsRemoved");
 
     QList<int> sizes;
-    sizes << 1000 << 4000 << 16000 << 64000 << 256000;
-    //sizes << 50000 << 100000 << 150000 << 200000 << 250000;
+    sizes << 100000;
 
     foreach (int n, sizes) {
         QStringList allStrings;