Add function to prune "dead" crons

[tollmanz]

There should be an easy way to unschedule crons that no longer have valid callback functions. This often occurs if a repeating event's callback function has a name change or is removed altogether.

[jrfnl]

I would not advocate for making this 'easy'.

I just added an indicator which checks & shows whether cronjobs have hooked in actions ([#40]) and found that even some of the core events don't always have their actions hooked in.

In other words, both core as well as plugins and themes might practice lean loading and only conditionally hook into their regular cronjob hooks (which are normally only scheduled on activate). The hooks would (rightfully) still be there, but the actions only hooked in if and when necessary.

In the next version there will be a way to - individually - remove cronjob events.

Adding a bulk action for events without hooks feels like it will lead to trouble when people who do not read warnings click it and let's be fair: how many users do (read warnings) ?

I suggest closing this issue.