fix user merge for mysql

mysql doesn't like simple subqueries in updates and deletes

refs CNVS-7032

Change-Id: Ibd3a4d51049fabacdffed2623d7ed5c13c6659b7
Reviewed-on: https://gerrit.instructure.com/28645
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>