Bug 17944 - Add Koha::ItemType->can_be_deleted and use it from admin/itemtypes.pl

Bug 17944 - Add Koha::ItemType->can_be_deleted and use it from admin/itemtypes.pl

Removed the sql code from Itemtypes.pm and replaced  it with DBIx
database query in the itemtypes.pl administrative script

Test plan:
1. In the staff interface, stage and manage MARC records for import

2. Try to delete an itemtype. If there are items of that itemtype in the
   database then a message telling you the number of items of that
   itemtype there are will be displayed.

3. Record that number

4. View the admin/itemtpes.pl script and confirm that there is sql code
   written in this file.

5. Apply this patch

6. View the admin/itemtypes.pl script and observe that there is no sql
   in this file. There is however DBIx code, for example
   $schema->resultset('Item')->search({ 'itype' => $itemtype_code} );
   which is searching for items with the itype value matching
   $itemtype_code value.

7. In the staff interface try to delete the same itemtype

8. Record the number of items there are with that itemtype in the
   resulting message

9. The numbers recorded in steps 3 and 8 should match showing that the
   DBIx code is working as intended

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>