In Registry Editor:
"HKEY_CURRENT_USER\Software\Blizzard Entertainment\WorldEdit".
Find an item called "Tool Windows" and delete it.
Be very careful that you only made changes for this intended part in Registry Editor, or else your system gets pretty much screwed up.
And remember to search before you post, there are plenty of tutorials that teach you how to make both pre-Reforged and Reforged WEs fully working at the same system.