How can I remove a locked post in the design forum caused by also entering the same post in the Premiere forum?

You can't! As soon as there are answers you can't modify posts on your own. 

A moderator moved simply to the premiere forum, but as in the meantime the post was duplicated there it got locked. No need to worry. I will add a link to the open discussion so that all users can find the life discussion when they see your original post.