Bluetooth: host: Remove reset of conn handle on disconnect

(Re)setting the connection handle in hci_disconn_complete
should not be done as the handle are used for
logging/debugging purposes after this, and makes it
impossible to lookup the handle of disconnected
connections.

The connection handle is set to 0 during bt_conn_new
in any case.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>