After working with multiple levels of TurboTax tech support, we came to the unpleasant conclusion that there is no way to delete the credit for AMT paid in the prior year and the only solution to the situation is to start anew. That means with a new, different, TurboTax account so none of the history from prior years' returns are imported. It took more work to file taxes this year, of course, but this approach solved the problem. (Tip: when creating a new TurboTax account, you can't use the same mobile phone number that you used with the first account.)
