LU-13814 osc: cleanup osc_completion

LU-13814 osc: cleanup osc_completion

Removing oap_cmd usage in osc_completion makes it easier to
remove osc page usage for DIO.

There's also no need for the osc_ap_completion wrapper, so
remove it.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ie72eecded73b0444a51b0273bf8913bfaee001bd
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52141
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Marc Vef <mvef@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>