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/34212
Lustre-commit: e763467ebe00913e8d03f855dc4b918b95099931

Change-Id: Ib80b0f69be008b4f895586dde35d1a5833a1a861
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/34496
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>