raid: no wiping when zeroing raid metadata device

raid: no wiping when zeroing raid metadata device

Currently lvm2 is not wiping signatures when creating 'metadata' volumes
and raid _rmeta was the only exception - so make the behavior consistent
with other metadata devices and drop wiping ATM.
Drop also some extra debug since they are now more explanatory in
wipe_lv() function.
Also note - although lvm2 now does not wipe signatures - the error
from such wipping used to be actually 'ignored' before wipe_lv()
started to return error (with recent commit) and raid creation
continued with 'unzeroed' metadata device.

TODO: Several issues to resolve:

1. We may want to flip to wipping with all LVs (in that case we need to
support passing --yet & --force).

2. Also we may want to clear whole metadata device - however current
function is also used for wipping i.e. snapshot COW device which
is likely not a good candidate for full device zeroing.
We may also need to think about better logic when extent size is
enforcing very large LVs, when only a small portion of LV is ever
being used.

3. Using TRIM instead of zeroing metadata device might be worth to
implement.

mm