Bluetooth: Make the function sco_conn_del have a return type of void

This makes the function sco_conn_del have a return type of void now
due to this function always running successfully and thus never
needing to signal its caller when a non recoverable internal failure
occurs by returning a error code to its respective caller.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>