Keep app hash integration conforming to protocol v4

Revert "Fix negative fee problem"
This reverts commit 2ce5ed2.

This is required to keep the app hash integration when processing older
blocks.

amo/amo.go

@@ -529,16 +529,6 @@ func (app *AMOApp) DeliverTx(req abci.RequestDeliverTx) abci.ResponseDeliverTx {
 	}
 
 	fee := t.GetFee()
-
-	if fee.LessThan(types.Zero) {
-		return abci.ResponseDeliverTx{
-			Code:      code.TxCodeInvalidAmount,
-			Log:       "negative fee",
-			Info:      "negative fee",
-			Codespace: "amo",
-		}
-	}
-
 	balance := app.store.GetBalance(t.GetSender(), false)
 
 	if balance.LessThan(&fee) {