Remove frozen-data-generation challenge

[danielmitterdorfer]

The challenge frozen-data-generation is not specific to frozen indices and can be replaced e.g. by index-logs-fixed-daily-volume. The only difference is that frozen-data-generation also force-merges the created index at the end. We can include the force-merge in index-logs-fixed-daily-volume as well but need to ensure that any Rally invocations that don't want the index force-merged, specify --exclude-tasks="type:force-merge" (see also elastic/rally#844).