cutils: Drop broken support for zero strtosz default_suffix

cutils: Drop broken support for zero strtosz default_suffix

Commit 9f9b17a4's strtosz() defaults a missing suffix to 'M', except
it rejects fractions then (switch case 0).

When commit d8427002 introduced strtosz_suffix(), that changed:
fractions are no longer rejected, because we go to switch case 'M' on
missing suffix now.  Not mentioned in commit message, probably
unintentional.  Not worth changing back now.

Because case 0 is still around, you can get the old behavior by
passing a zero default_suffix to strtosz_suffix() or
strtosz_suffix_unit().  Undocumented and not used.  Drop.

Commit d8427002 also neglected to update the function comment.  Fix it
up.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>