LU-13475 utils: disable lfs_migrate rsync and warning

LU-13475 utils: disable lfs_migrate rsync and warning

The --rsync option is no longer enabled by default for fallback if
'lfs migrate' fails for some reason, and is mandatory for rsync usage.
The warning message and "-y" option of lfs_migrate is no longer needed
if rsync is not used, and is only shown if --rsync is used.

Remove the LFS_MIGRATE_RSYNC_MODE variable that was used for tests
and instead pass the "--rsync" option directly when needed.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I70b70d969f2dc8b4836c6c7692e6a73a0e2540e5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/40614
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Vitaliy Kuznetsov <vkuznetsov@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>