LU-16748 llite: update comment of ll_swap_layouts_close

LU-16748 llite: update comment of ll_swap_layouts_close

mdc_close_layout_swap_pack() does not exist, and the lease lock
handle is release in mdc_close_intent_pack(). This patch updates
the comment.

Also, mdt_close_swap_layouts() does not exist. This patch removes
its declaration.

Test-Parameters: trivial
Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: Iecd1754f627803e85f54a91d648e87e235106bd7
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50660
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>