The CWalletDB::FindWalletTx method was patterned after CWalletDB::LoadWallet, where fNoncriticalErrors is used when a tx check fails in ReadKeyValue.
Since FindWalletTx is only used by methods which are zapping txs, it makes sense that ReadKeyValue is not called and the tx is not checked, so I think that deleting the unused fNoncriticalErrors boolean variable and its conditional statement is appropriate.