Bug 36195: Replace POST in itemsearch.tt with GET

We retrieve a list of records for DT, it does not need to be a POST
request.

Test plan:

1. Try an item search
2. The page loads but pops an alert that says "403: Forbidden" and table stays empty
3. Apply patch
4. Try an item search again and the table loads results

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>

koha-tmpl/intranet-tmpl/prog/en/modules/catalogue/itemsearch.tt

@@ -478,7 +478,6 @@
                 "processing": true,
                 "ajax": {
                     url: '/cgi-bin/koha/catalogue/itemsearch.pl',
-                    type: 'POST',
                     data: function ( d ) {
                         for (i in params) {
                             d[params[i].name] = params[i].value;