Bluetooth: att: Remove att use of bt_l2cap_chan_send_cb

EATT is the only user of `bt_l2cap_chan_send_cb`, and not necessary. We
can maintain the same functionality without it.

Instead of passing and storing the callback into l2cap, we can store it
in a callback queue in the ATT bearer struct.

This will allow us to remove that internal API later, in order to
streamline the l2cap API.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>