don't use write_attribute for student count

refs CNVS-15830

test plan:
 * courses list for root account should still show student count and
  teacher names

Change-Id: Ib65af09e3e06f7a0176fcc517d579b3ae4204053
Reviewed-on: https://gerrit.instructure.com/41804
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>