Remove buggy, unused function get_leaders_from_usergroup

Recent editing and testing of get_leaders_from_usergroup() uncovered an
unrelated bug which renders the function useless (it returns all group
members, not just the leader). It wasn't previously caught because
Evergreen has no apparent references to this function. This commit
removes the unused function.

Signed-off-by: Remington Steed <rjs7@calvin.edu>
Signed-off-by: Dan Wells <dbw2@calvin.edu>