.The two previous answers are for removing all of Exchange from your domain. Since you just want to remove one server, but keep the rest of the organization intact, you should look at
Summarising this: you reset Valkerie's Active Directory machine account, build a new clean server called Valkerie and join it to the domain, and then run Exchange setup on Valkerie in a special mode that conigures it in exactly the same way Valkerie was configured.
That gets you a server that AD and Exchange think is Valkerie reborn: then you can do a clean uninstall of Exchange on her and AD will clean itself up properly this time.
Lastly, you don't need to worry about this since Valkerie was only a UM server, but in case anyone else is reading this: if the server you are recreating was part of a cluster, then the procedure is slightly different - investigate the RecoverCMS option instead.