In a previous post; Item Site Assignment, the results of deleting item-site assignment were thoroughly illustrated. The bottom line is, all you purchase receipt layers work and details for this item-site relation will be removed.
Although History transactions remain, the fact of deleting purchase receipt layers itself is not an option for most of GP Clients.
In GP 2013, you can inactivate the site so you no longer can perform any transactions on the it.
Mahmoud M. AlSaadi