Drop obsolete "USB stick cannot start after interrupting persistence wizard" warning

Or at least we have something pretending this in our installation instructions:

If you close the assistant before the creation finishes, you might not be able to start Tails from this USB stick anymore.

I did some tests and couldn’t reproduce this so at least it means that it doesn’t happen all the time.

Help desk: Do you remember seeing that in the last year or so?
Foundations: Do you think this bug could still occurs?

Parent Task: #14548

Original created by @sajolida on 15100 (Redmine)