Just delete the 2018 refund. If you know it's not taxable because you took the standard deduction, there is no need to enter it.
Even when you do enter a state refund that's not taxable, you are not reporting that you received it. It doesn't show up anywhere on your tax return. It just stays in an internal TurboTax worksheet. So if you know it's not taxable, there is no need to enter it. You could just as well delete the 2019 refund also. It won't make a bit of difference in what is reported on your tax return.
