Bluetooth: 6lowpan: Fix using chan->conn as indication to no remote netdev

Bluetooth: 6lowpan: Fix using chan->conn as indication to no remote netdev

b66774b48dd9 ("Bluetooth: L2CAP: Fix UAF in channel timeout by holding
conn ref") don't reset the chan->conn to NULL anymore making the bt#
netdev not be remove once the last l2cap_chan_del is removed.

Instead of restoring the original behavior this remove the logic of
keeping the interface after the last channel is removed because it
never worked as intended and the l2cap_chan_del always detach its
l2cap_conn which results in always removing the channel anyway.

Fixes: b66774b48dd9 ("Bluetooth: L2CAP: Fix UAF in channel timeout by holding conn ref")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>