NO_CLEAN: Hide delete-old output.

It is full of distracting noise about UPDATING and may confuse
the user about what is actually being deleted. It is also
possible to have directories removed on every run with
use of WITHOUT_ knobs that the mtree files do not
account for and for which the directories are incorrectly
in OLD_DIRS currently.

X-MFC-With: r321443
MFC after: 1 month
Sponsored by: Dell EMC Isilon