Removing Task Services should uninstall the Task Service from the Site Server. The Task Service Uninstall task is used to remove Task Services from the Agent UI on the Site Server. This task does not remove the Task Service and stays in the Agent UI.
Task service uninstall doesn't uninstall task server plugin
8.5 through 8.6 RU2, issue is resolved in 8.6 RU3
Download the attached xml file and rename it back to ServerUninstall.xml. Put this file on the Notification Server in <InstallDrive>:\Program Files\Altiris\Notification Server\NSCap\bin\Win32\X86\Client Task Management\Server Uninstall Package.
Open Windows Task Scheduler and run these 2 tasks: NS.Package Refresh and NS.Package Distribution Point Update
On the Site Server, Update Configuration on the Agent UI and wait for the task to execute again.
Alternatively, it is possible to uninstall Task Services manually by Copying the ServerUninstall.xml (don't rename it) to ConfigFull.xml, and then executing the Uninstaller.exe. You will see Task Services uninstall.