don't delete from params in ContentImportsController

refs CNVS-16685

Change-Id: I4cd92ab8078ae1709b470ab6fb4cc1e410282044
Reviewed-on: https://gerrit.instructure.com/44290
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>