Bluetooth: `hci_driver.h`: Remove `bt_recv_prio`

Remove `bt_recv_prio` from the HCI driver interface. It's primary
purpose was to be used in conjunction with `CONFIG_BT_RECV_BLOCKING`,
which has been removed.

`hci_raw.c` provided a adapter implementation to make drivers
implementing the `CONFIG_BT_RECV_BLOCKING` interface useable with
`CONFIG_BT_HCI_RAW`. But since `bt_recv_prio` is being removed from the
HCI driver interface, drivers needing the adapter should copy the
adapter implementation as private internals.

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