Open the Print Servers branch, <computer name>, Drivers
Select the Amyuni Document Converter 2.51 entry (don't choose any other!!) - if this driver isn't present, then it was fully removed by the Attach Plus uninstall
Right click, Remove Driver Package...
Click OK, then Yes
Wait for the green 'Drive package information collection' progress bar to go all the way across the dialog
Click Delete
Wait for the green 'Driver package deleted' progress bar to go all the way across the dialog again
Click OK
Click Close
At this point, Attach Plus and it's drivers have been removed from the system.