remove description from the sis assignments api

test plan:
 * query the sis assignments api for an account/course
   * /api/sis/accounts/:account_id/assignments
   * /api/sis/courses/:course_id/assignments
 * the description field should not be returned for any assignment

fixes CNVS-22513

Change-Id: I2d679a3b7eedf4f4892ef5e7d1b93007e7500c7a
Reviewed-on: https://gerrit.instructure.com/60861
Reviewed-by: Brad Humphrey <brad@instructure.com>
Tested-by: Jenkins
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Mark Severson <markse@instructure.com>

app/controllers/sis_api_controller.rb

@@ -46,7 +46,6 @@ class SisApiController < ApplicationController
   #       "id": 4,
   #       "course_id": 6,
   #       "name": "Assignment Title",
-  #       "description": "Assignment Description",
   #       "created_at": "2014-12-01T17:00:00Z",
   #       "due_at": "2015-01-01T17:00:00Z",
   #       "points_possible": 100,

lib/api/v1/sis_assignment.rb

@@ -20,7 +20,7 @@ module Api::V1::SisAssignment
   include Api::V1::Json
 
   API_SIS_ASSIGNMENT_JSON_OPTS = {
-    only: %i(id description created_at due_at points_possible integration_id integration_data).freeze,
+    only: %i(id created_at due_at points_possible integration_id integration_data).freeze,
     methods: %i(name).freeze
   }.freeze