Bluetooth: Remove useless sk variable in l2cap_le_conn_ready

The sk variable is of quite little use since it's only used to simplify
access in the two bt_sk() calls.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>