preload context for destroy_stream_item_instances
preload context for destroy_stream_item_instances

this removes an n+1

test plan
 - specs should pass

flag=none

Change-Id: I483657cb3b09ed92bb9e153ebfdb848504c759aa
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/264184
Reviewed-by: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>
QA-Review: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>
Product-Review: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms