Bug 36931: Fix label-item-search.pl by removing CSRF requirement

Bug 36931: Fix label-item-search.pl by removing CSRF requirement

This change converts a stateless POST into a GET, so that the paging
of the label-item-search.pl results works again.

Test plan:
0. Apply the patch and restart Koha
1. Go to http://localhost:8081/cgi-bin/koha/labels/label-edit-batch.pl?op=new
2. Click "Add item(s)"
3. Add '05/01/2000' in "Added on or after date:" and click "Search"
4. Page through the results
5. Rejoice

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>