LU-14694 mdt: do not remove orphans at umount

LU-14694 mdt: do not remove orphans at umount

as it's very likely that another MDT is being umounted as well
and such a removal can get stuck if the object being removed
is a striped directory.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I0417b1b4447887e166c144605bbfa3249126eacd
Reviewed-on: https://review.whamcloud.com/43783
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>