Sometimes it is necessary for a teacher to remove already submitted work from the list of assignments. Possibly due to someone submitting the wrong work, or to completely eradicate someone's presence in the activity. For me it was a client that had users submitting work according to the wrong instructions. Leading to have an activity in the course with mixed submissions. A fix would have been to be able to delete the individual submissions. Both files and feedback and completion statuses.
I have submitted tracker issue MDL-37237 and put in some work to create the feature. If you want this feature in the latest version, please feel free to vote on it. If you want this feature in your older version; I have put up patches for 2.5 through to 2.7 as well.
Vote yes here: