sorry for the late reply on this, I assume you already resolved this by now.
you can do a manual removal by removing the objects via ADSIEdit.msc, though there is risk there to be sure you have a good backup of the domain. (global catalog server).
then just find the old server name under
configuration store -> services -> Microsoft Exchange -> First Organization -> Administrative Groups -> Exchange Administrative Group -> Servers
before you do this be SURE you have a backup of a DC.