Exclude group tutors and admins from showing under "Members without a submission to the group"

When you set up a group that allows submissions (need to have admin or staff permissions in Mahara), you see all group members.

However, it would be better not to show group tutors and admins as they usually do not need to submit portfolios.

Instead of making it a configurable option, it would be sufficient to simply not show group tutors and admins.

Group tutors only come into play when a group is set up as "course" group. Thus, both sets of groups will need to be tested.

They still need to show up under those people that have submitted portfolios if they do do so at some stage.

1. Create a standard group that allows submissions and set the group settings to "controlled".
2. Add 10 people to the group.
3. Make one of them a group admin.
4. Go to the group homepage and see who is listed under "Members without a submission to the group".
Expected result: You only see the regular group members.
Actual result: You also see the 2 group admins.

5. Change the group settings to "course" group, which now allows you to set up a group tutor.
6. Change the role of a regular group member to that of a tutor.
7. Go to the group homepage and see who is listed under "Members without a submission to the group".
Expected result: You only see the regular group members.
Actual result: You also see the 2 group admins and the tutor.