Bluetooth: Remove redundant call to rfcomm_send_disc

In rfcomm_session_del() remove the redundant call to
rfcomm_send_disc() because it is not possible for the
session to be in BT_CONNECTED state during deletion
of the session.

Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>