osd/ReplicatedPG: skip promote for DELETE

osd/ReplicatedPG: skip promote for DELETE

If an op starts with DELETE there is no need to promote the old content
from the base tier.  Note that this only works if the FAILOK flag is
set.  Otherwise, we need to know whether the object existed or not to
return either 0 or -ENOENT.

Signed-off-by: Sage Weil <sage@inktank.com>