qapi: Drop dead dealloc visitor variable Commit 0b9d8542 added StackEntry.is_list_head, but forgot to delete the now-unused QapiDeallocVisitor.is_list_head. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com> Message-Id: <1454075341-13658-4-git-send-email-eblake@redhat.com> Signed-off-by: Markus Armbruster <armbru@redhat.com>