Bug 21182: Remove acqui/check_duplicate_barcode_ajax.pl

Bug 21182: Remove acqui/check_duplicate_barcode_ajax.pl

It has been added by
  commit 327be442bd6e63e5ff8c0a5e2461ae836e8deffb
  Bug 6963: Corrects a problem when a new order is created with a duplicate barcode.

But its last call removed since:
  commit eefc774e274ce8167f1063be5f9204a0d5cf5650
  Bug 7178: Acquisition item creation improvement

Test plan:
  git grep check_duplicate_barcode
should not return anything

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>