Remove label from forum posts by global staff

The motivation for this change is performance. The forums UI code gets
the list of users for each role and renders the staff label based on
those lists. The list for the staff role is expensive to compute because
there is no index on the is_staff attribute, and we cannot create one
because the User model is built into django.

Users with is_staff=True are still assigned the Moderator role upon
enrolling in a course, so this change will have no practical effect
except that a user who is granted staff privileges after enrolling in a
course will have to be made a Moderator in order for their posts to be
labeled.

Additionally, the UI did not use the list of users with the Student
role, so that list has been removed as well.
Merge pull request #1570 from edx/gprice/forum-staff-label

Remove label from forum posts by global staff