jbd2: remove unused transaction->t_private_list

After we remove ext4 journal callback, transaction->t_private_list is
not used anymore. Just remove unused transaction->t_private_list.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20241218145414.1422946-3-shikemeng@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>