Remove final portion of the TxOut item local encryption

Follow up on !2821 (merged) that removed the encryption pathway for the local storage of TxOut items.

Resolves !1632 (merged) which also contains the rationale behind the removal, as well as removing the sole usage of MD5 in the codebase.

Not to be merged until we are sure all active nodes have churned through the TxOut kv-store with the new pathway in !2821 (merged).

bifrost/observer/observe.go

@@ -121,11 +121,14 @@ func (o *Observer) Start() error {
 	return nil
 }
 
-// ObserveSigned is called when a tx is signed by the signer and returns an observation that should be immediately submitted. Observations passed to this method will be cached in memory and skipped if they are later observed in the mempool or block.
-func (o *Observer) ObserveSigned(txIn types.TxIn) {
-	// add all transaction ids to the signed tx out cache
-	for _, tx := range txIn.TxArray {
-		o.signedTxOutCache.Add(tx.Tx, nil)
+// ObserveSigned is called when a tx is signed by the signer and returns an observation that should be immediately submitted.
+// Observations passed to this method with 'allowFutureObservation' false will be cached in memory and skipped if they are later observed in the mempool or block.
+func (o *Observer) ObserveSigned(txIn types.TxIn, allowFutureObservation bool) {
+	if !allowFutureObservation {
+		// add all transaction ids to the signed tx out cache
+		for _, tx := range txIn.TxArray {
+			o.signedTxOutCache.Add(tx.Tx, nil)
+		}
 	}
 
 	o.globalTxsQueue <- txIn