Bluetooth: Host: Do not require the ops->sent callback

The L2CAP channel ops->sent callback was required to call the
l2cap_chan_sdu_sent callback.

Previously, the only difference between the l2cap_chan_sdu_sent and
l2cap_chan_seg_sent callbacks was that l2cap_chan_sdu_sent called
ops->sent. This is no longer true and l2cap_chan_sdu_sent should always
be called when an SDU is sent.

Signed-off-by: Herman Berget <herman.berget@nordicsemi.no>