DERBY-3155: Remove spurious comma from grammar production for multiple WHEN [NOT] MATCHED clauses in MERGE statement; tests ran cleanly on derby-3155-02-ag-fixParserWarning.diff.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1524432 13f79535-47bb-0310-9956-ffa450edef68