If a server, running Active Directory Connector crushes or is decommissioned from the network without first uninstalling ADC you can no longer remove this ADC through the ADC snap-in. You can see it in the snap-in but there is no option to
delete it.
Here you can see how to remove such ADC by using ADSIEdit:

Step 1 In this screencast you can see how to use ADSIEdit to remove an Active Directory Connector.