\core\task\stats_cron_task and $CFG->statsruntimestarthour are contradictory

When statistics processing was converted to a scheduled task we did not get rid of statsruntimestarthour setting at the same time. This is confusing for administrators - because altering the scheduled task doesn't really have much of an effect. Hat tip to mrverrall for pointing out this problem.

I suggest we get rid of statsruntimestarthour and instead move to the proper schedule task mechanism - that would also allow you to run it on demand through CLI.

Some additional thoughts:

1. We need to take special care of the scheduled task to move it away from a once every minute schedule. I think we'd need to force a reset of the schedule in upgrade. People may have customised it (for now gain)
2. I don't necessarily think we need to observe statsruntimestarthour - so long as we document this heavily in the release notes.
3. We should review the strings to see which ones make sense in scheduled task world