docs: Remove callback prop example from 02-runes.md

[leoj3n]

$: and $effect are not related to events and callback props.

Currently this example doesn't really make any sense AFAICT. It is not clear what it is trying to exemplify that is not already covered by the first to "Don't do this; Do this instead" examples showing using $derived(.by) for setting $state based on other $state. It looks like callback props are related to events, nothing really to do with bound values that are read in $:. Perhaps this was meant to be a third $derived example that shows use with bind: or something along those lines?

Please correct me if I'm wrong; perhaps this example exists for a reason I don't understand.

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[dummdidumm]

The example shouldn't be removed, this is a case where using derived wouldn't work because the state can not be derived. That said, the example doesn't make it that clear, so we should probably see how we can clarify that.

[Rich-Harris]

To be honest I don't really understand this example either. It seems like it's kinda covered by the more general 'don't use $effect to synchronise state' guidance immediately above. I would go further and remove the following section too — opened #10750.

[benmccann]

$: and $effect are not related to events and callback props.

Without commenting on this specific example, which I haven't looked at closely, you can avoid using $effect by leveraging callback props. This is quite separate from events. For an example, see #9944 (comment).