Bug 11460: Remove MARC21 default mapping from itemcallnumber system preference

Bug 11460: Remove MARC21 default mapping from itemcallnumber system preference

The mapping in the itemcallnumber system preference is only valid
for MARC21 libraries using DDC classification, but not for libraries
using UNIMARC or other classification.

This patch suggests to remove the default mapping and start new installations
with an empty value.

To test:
- On an emtpy database
- Apply patch
- Run the web installer, selecting either MARC21 or UNIMARC
- Verify that the itemcallnumber system preference is empty
- Verify the description now also include examples for UNIMARC

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>