fix: prune cmd should disable async pruning

[technicallyty]

Description

Closes: #23810

fix from #22656

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[aljo242]

If this is the main place we use !rs.iavlSyncPruning should we just have an option that is rs.iavlAsyncPruning instead?

[technicallyty]

I'm not sure what you mean

[aljo242]

lets discuss sync

[technicallyty]

ok dug more here, this has to do with default values. we shouldn't really ever expect anyone to set this value, and i've updated comments to reflect that here.

that means the default value false will always take effect, enabling the async behavior to be default.

in short, we want the naming of the flag to reflect what the default value implies. explicit setting of the value should only ever be done from the prune cmd. i tried thinking of other ways to set this value, but i believe this is the most straightforward, low friction way to do it.