Bug 16672 - Add ability to remove expired holidays from special_holidays

Bug 16672 - Add ability to remove expired holidays from special_holidays

It would be very helpful if the cleanup_database.pl script had the
ability to delete holidays from the special_holidays table there were
older than a given number of days in the past.

Test Plan:
1) Apply this patch
2) Create some unique holidays in the past of varying ages
3) Test the new switch '--unique-holidays DAYS' for cleanup_database.pl
4) Verify only holidays older then the specified number of days are removed

NOTE: The language 'unique holdays' is used in the interface to match
      it's use in the staff web interface.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>