Bluetooth: Move SCO timeout constants into net/bluetooth/sco.c

There is no external user of the SCO timeout constants and thus
move them into net/bluetooth/sco.c where they are actuallu used.

In addition just remove SCO_CONN_IDLE_TIMEOUT since it is unused.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>