If you have one thread in forum "A" and then you move it to forum "B" leaving a redirection notice you're able to delete the redirection notice, which is good. However if you soft delete the redirection notice in forum "A" instead of completely disappearing from the list it's being displayed as a deleted item thus having the same item in forum "B" might be confusing.
I think this should work as it does in vBulletin where if you attempt to delete a redirection notice it gets permanently removed without asking you anything.
A suitable suggestion would be to ask for a feature where we - as administrators could remove the redirect in question - like vBulletin, of course with a different semantic.
I know what this thread is about. I've had this 'problem' er... situation before. I moved a thread to forum b in order to "delete" the thread, and I forgot to click "don't redirect" button. So, I had to move it back into the original forum and then move it back to the trash forum. By the end of the move, the redirect is gone when I put it back to where it originally was, so in the move to the trash forum, I click "don't redirect." Finished.