You delete a system activity either via the Flowchart Editor (See chapter Delete steps or via Steps (See chapter Delete steps). In the former case, right click the system activity symbol in Flowchart Editor. Then, in the cursor context menu, click Delete step to delete the system activity.
In the latter case, select Steps in the Designer navigation menu. The step overview appears. Check the box next to the system activity you wish to delete. Click on Delete in the action bar. The system activity will be deleted.