Bluetooth: host: Remove ATT modification of L2CAP conn pointer

Remove ATT modifying the L2CAP channel state by unassigning the
the connection pointer on timeout.
Unassigning this pointer does not prevent the ATT channel from receiving
since bt_l2cap_recv does not inspect this pointer before calling the
channel receive function.
This prevented the disconnected callback from being called on the
channel after the channel had timed out, but since the disconnected
callback now handles this case this workaroun is no longer needed.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>