drop unused columns from asset user accesses table

refs CNVS-15776

test plan
 - migrations should run

Change-Id: I38f694782ed1733b83271bc1369c318183ed0120
Reviewed-on: https://gerrit.instructure.com/41770
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>