remove reference to missing plugins css package

fixes CNVS-12836

test plan:
- visit the plugins index

Change-Id: Ie37dbc5ef5f1f00860b9071c21ec49c5fc852d1c
Reviewed-on: https://gerrit.instructure.com/34261
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>