Bug 30947: Simplify CanBookBeIssued date handling

Bug 30947: Simplify CanBookBeIssued date handling

1) This removes support for passing string dates to CanBookBeIssued. The
function didn't publicly even document support for string dates, only
DateTime objects.

2) We get a $duedate always at least from CalcDateDue so having

 $issuingimpossible{INVALID_DATE} = output_pref($duedate);

was unneccesary and thus removed.

3) The check "duedate cannot be before now" was needlessly complex: if
the due date really cannot be before now we should check seconds too
and warn the librarian! Thus the truncation to minutes can be dropped
safely.

To test:
 1) prove t/db_dependent/Circulation.t
 2) prove t/db_dependent/Illrequests.t
 3) Enable OnSiteCheckouts and disable SpecifyDueDate syspref. Create
 on-site checkout for any patron and verify the due date is your
 current date at 23:59, you can check the exact minute with sql:
  > select * from issues

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>