Bluetooth: host: remove TX thread

We don't need the TX thread anymore.

Generalizing the pull-based architecture (ie. `tx_processor`) to HCI
commands makes it possible to run the whole TX path from the the system
workqueue, or any workqueue really.

There is an edge-case, where we call `bt_hci_cmd_send_sync()` from the
syswq, stalling the system. The proposed mitigation is to attempt to drain
the command queue from within `bt_hci_cmd_send_sync()`.

My spidey sense tingles however, and it would be better to just remove the
capability of calling this fn from the syswq. But doing this requires
refactoring a bunch of synchronous procedures in the stack (e.g. stack
init, connection establishment, address setting etc), dragging in more
work. I will do it, but in a later patch.

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