qapi: Clean up after removal of simple unions Commit 4e99f4b12c0 (qapi: Drop simple unions) missed a bit of code dealing with simple union branches. Drop it. Signed-off-by: Markus Armbruster <armbru@redhat.com> Message-Id: <20230316071325.492471-4-armbru@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com>