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>

qapi/qapi-dealloc-visitor.c

@@ -29,7 +29,6 @@ struct QapiDeallocVisitor
 {
     Visitor visitor;
     QTAILQ_HEAD(, StackEntry) stack;
-    bool is_list_head;
 };
 
 static QapiDeallocVisitor *to_qov(Visitor *v)