Bluetooth: L2CAP: remove usage of `bt_l2cap_send_cb` by BR/EDR

Isolates BR/EDR's l2cap use from LE (most of l2cap.c).

Why? Because I'm refactoring l2cap.c and don't want to accidentally
break BR/EDR support, as there are very few tests in CI.

This way, once `bt_l2cap_chan_send` hands control to l2cap_br.c, it will
not call back into l2cap.c, it will instead be a parallel layer going
directly to conn.c.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>