IB/iser: Remove buggy back-pointer setting

The iscsi connection object life cycle includes binding and unbinding
(conn_stop) to/from the iscsi transport connection object.  Since
iscsi connection objects are recycled, at the time the transport
connection (e.g iser's IB connection) is released, it is not valid to
touch the iscsi connection tied to the transport back-pointer since it
may already point to a different transport connection.

Signed-off-by: Or Gerlitz <ogerlitz@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>