DERBY-5313: Assert failure with CASE expression in GROUP BY clause

backported change #1575866 from trunk to 10.10 branch, had to do hand
    merging due to jdk5 code differences in 10.11.

Remove assert that doesn't expect expressions in the GROUP BY list.



git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/branches/10.10@1629018 13f79535-47bb-0310-9956-ffa450edef68