simplify can_do

adheres_to_policy has equivalent caching internally now, so no need
to bother with externally

also, OpenObjects went the way of the dodo a long time ago
(StreamItems always reconstitute the actual AR object)

Change-Id: I9f2e04700c8897d0fe539a58dcbd6ec829e6f023
Reviewed-on: https://gerrit.instructure.com/36310
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>