ceph-volume: do not fail when trying to remove crypt mapper

ceph-volume: do not fail when trying to remove crypt mapper

In a containerized context, at some point, need to run `simple scan` on a device
from a separate container (not the existing and running corresponding container
to that device), but this can't work because when it tries to remove the
mapper which is still in use by the corresponding running osd container,
it fails.
This can be a bit more permissive and simply throw a warning.

Closes: https://tracker.ceph.com/issues/41392
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>