fix CollectionAssociation#distinct for rails 4

the signature of the method changed, so we can simplify

Change-Id: Ie579809d3dfe6c93383eb5edbae16b1c7f4b07fa
Reviewed-on: https://gerrit.instructure.com/42071
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>