When the need arises to remove a Site System from DP the first thing you need to do is remove all of the roles from the Site System. However, often when you are doing this you find that the ‘ConfigMgr Component Server’ role persists which stops you from deleting the system.
Generally just leaving this for a few hours, or a couple of days works however if you are in a rush (as we all usually are!) then a quick fix is to edit another role on another site system, e.g. edit the deletion policy on a State Migration Point role. Wait for 10-15 seconds after doing that and the component server should remove on your other site system and you should get the option to delete.
As per this thread;