LU-9706 dt: remove dt_txn_hook_commit()

LU-9706 dt: remove dt_txn_hook_commit()

it's not used and it's not safe as dt_txn_callback_del()
and dt_txn_callback_add() can race with commit callbacks.

Lustre-change: https://review.whamcloud.com/#/c/34212/

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I1744aaa621e28cb3f7e812db5695aa42e8d596cd
Reviewed-on: https://review.whamcloud.com/34274
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>