correct department statistics api documentation

we had to remove discussion_replies and submission (previously) because
they were too slow.  this commit removes references to them in the api
docs and also the underlying methods that were being called, but are not
longer used

closes CNVS-17634

test plan:
- check that the api documentation fields for department statistics
  match what is actually returned

Change-Id: I68754eddd329768a890029113c14e1a4078bedc4
Reviewed-on: https://gerrit.instructure.com/46201
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>