Don't mention confirmation dialog when doing a manual upgrade from Tails Cloner

https://tails.net/upgrade/clone/ has "Read the warning message in the confirmation dialog" and "Click Delete All Data and Install to confirm" steps. A user reported this step did not match reality: they did not see this message when upgrading. I've confirmed by testing. And a look at the code & test suite suggests this message is only displayed when installing (on a stick with no Persistent Storage), not when upgrading.