Bluetooth: Remove unneeded check in hci_disconn_complete_evt()

According to b644ba33 (patch that introduced HCI_CONN_MGMT_CONNECTED
flag), the HCI_CONN_MGMT_CONNECTED flag tracks when mgmt has been
notified about the connection.

That being said, there is no point in calling mgmt_disconnect_failed()
conditionally based on this flag. mgmt_disconnect_failed() removes
pending MGMT_OP_DISCONNECT commands, it doesn't matter if that
connection was notified or not.

Moreover, if the Disconnection Complete event has status then we have
nothing else to do but call mgmt_disconnect_failed() and return.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>