leave context host for attachment_json in reports

attachment_json will do this for you.
this prevents double  context information from the
merge of options in attachment_json

refs CNVS-10842

test plan
 - run report with canvas on port 3000
 - report object response should have the correct
   attachment download url

Change-Id: I58caffede266dee157afab63fd82a0a4a4bd5213
Reviewed-on: https://gerrit.instructure.com/40638
QA-Review: Dave Jungst <dave@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>