I can't see how this would trigger in the current codebase, so it's not a *current* safety issue,
but I can very well see it becoming on in the future if downstream code doesn't handle the passing
of pruned transactions correctly. I think the safe/good choice would be to remove this skip now
that all transactions that pass into the mempool are supposed to be unpruned. And for all in-block
txs, `check_tx_inputs()` isn't called for checkpointed blocks, and we sync pruned blocks only if
syncing checkpointed blocks.
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