Bug 18928: Remove '(not set)'

Bug 18928: Remove '(not set)'

The "(not set)" part must be removed IMO, I am not sure I understand its goal.
Now that we have 1 entry per rule (and not 1 DB row for the whole "Default
checkout, hold and return policy for Centerville" table), it will be harder
to handle it correctly. Not impossible, but if not really needed I would prefer
to just remove it. If a rule is not set, the cell is empty or the dropdown list
displays the option "Not set", which seems to correct the problem.

Signed-off-by: Agustin Moyano <agustinmoyano@theke.io>
Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>