Fixed: The system still consider the customer_ids, even after field is empty on table customer_user (bug#9098).
Squashed commit of the following:

commit b48716c30455bde2cd57e9145b151405c9771b8f
Author: s7design <otrs@s7designcreative.com>
Date:   Sat Apr 5 17:12:00 2014 +0200

    [Bug 9098] The system still consider the customer_ids, even after field is empty on table customer_user

    Added code for delete cache for CustomerIDs