Fix a bug reported by Chris Snow at https://issues.apache.org/jira/browse/OFBIZ-2847 for "Create New Inventory Item - user entered Inventory Item Id overwritten with system generated one"

It does not make sense to allow entering an inventoryItemId. It is and must be generated by the system anyway (to prevent collisions)

git-svn-id: https://svn.apache.org/repos/asf/ofbiz/trunk@1552908 13f79535-47bb-0310-9956-ffa450edef68