[Closed by !2722 which includes it] Use GetConsensusSigners for pre-confirmation-counting observed txin consensus slashing as well

[V133-specific]

[Closed by !2722 (merged)
'Slash non-signers upon consensus'
which includes it.]

Intended to close #1639 (closed)
'!2994's implementation of GetConsensusSigners is incomplete'.

Edit: This is also now closable by !2722 (merged) 'Slash non-signers upon consensus'.

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For more details:
handler_observed_txin.go already uses voter.GetConsensusSigners()... for after-confirmation-count consensus unslashing ,
but still uses voter.Tx.GetSigners()... for before-confirmation-count consensus unslashing (which can miss transactions which contributed to that consensus while having minor differences from the Tx),
so the change made is to have both cases use voter.GetConsensusSigners()... .