LU-16518 lod: remove unused dt_object_qos_mkdir()

LU-16518 lod: remove unused dt_object_qos_mkdir()

This function was rendered obsolete in a previous
patch, but was not removed.

Fixes: c1d0a355a6a6 ("LU-12624 lod: alloc dir stripes by QoS")
Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I33ce5a5b745bff7414df8aa04ecf72d68cf8f715
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58351
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>