speed up & simplify rails controller for newfiles

fixes:  CNVS-15789

we don't need to do this check if a user can
"read_contents" of the root folder of the course
because if they can't the api request to get it
will just 401.

Change-Id: I4787bb8c3340f9215f7bfadf6e708e7c29a38aaa
Reviewed-on: https://gerrit.instructure.com/41705
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>