Bluetooth: Simplify L2CAP Streaming mode sending

As we don't have any error control on the Streaming mode, i.e., we don't
need to keep a copy of the skb for later resending we don't need to
call skb_clone() on it.
Then we can go one further here, and dequeue the skb before sending it,
that also means we don't need to look to sk->sk_send_head anymore.

The patch saves memory and time when sending Streaming mode data, so
it is good to mainline.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>