remove duplicate argument in content migration API

settings[file_url] was duplicated, as was source_course_id

Fixes SIS-279

Change-Id: Ied36085cfdb311c666a0d8c792ab475aa0452630
Reviewed-on: https://gerrit.instructure.com/34981
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Eric Adams <eadams@instructure.com>
Product-Review: Eric Adams <eadams@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>