v2.1: blockstore: mark slot as dead on data shred merkle root conflict (backport of #3970)

[mergify]

We need to mark the slot as dead to ensure that the bank is completed

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This is an automatic backport of pull request #3970 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 5564a94 has failed:

On branch mergify/bp/v2.1/pr-3970
Your branch is up to date with 'origin/v2.1'.

You are currently cherry-picking commit 5564a941ae.
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Unmerged paths:
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	both modified:   ledger/src/blockstore.rs

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