It is better to let the RunBenchmarks(), report() decide
whether to actually *only* output aggregates or not,
depending on whether there are actually aggregates.

It's subtle indeed.

Previously, `BenchmarkRunner()` always said that "if there are no repetitions,
then you should never output only the repetitions". And the `report()` simply assumed
that the `report_aggregates_only` bool it received makes sense, and simply used it.

Now, the logic is the same, but the blame has shifted.
`BenchmarkRunner()` always propagates what those benchmarks would have wanted
to happen wrt the aggregates. And the `report()` lambda has to actually consider
both the `report_aggregates_only` bool, and it's meaningfulness.

To put it in the context of the patch series - if the repetition count was `1`,
but `*_report_aggregates_only` was set to `true`, and we capture each iteration separately,
then we will compute the aggregates, but then output everything, both the iteration,
and aggregates, despite `*_report_aggregates_only` being set to `true`.