LU-16595 test: save one second in wait_destroy_complete()

LU-16595 test: save one second in wait_destroy_complete()

In wait_destroy_complete(), there is no need to wait another 1
second when all in flight destroys finish.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I351616ecf261f1e77c3f8d61f5541a51e327fa83
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50144
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>