Bug 29495: Drop issue_id constraint from return_claims

Bug 29495: Drop issue_id constraint from return_claims

This patch removes the issue_id constraint from return_claims.

Due to the nature of our dual table approach to checkouts/old_checkouts
we can't safely hae this constraint and not lose data.  Prior to this
commit, when an item is checked in we move the checkout from checkouts
to old_checkouts.. this therefore triggers the delete of the issue_id
from the return_claims table as described by the foreign key constraint.

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>