Bluetooth: host: remove `CONFIG_BT_RECV_BLOCKING`

This config selects a variant of the HCI driver interface that spills
out host internals unto the drivers and even the Zephyr controller. It
will now be removed in favor of driver interfaces that hide the
internals of the host.

The new default is `CONFIG_BT_RECV_WORKQ_BT`.

Any references to the removed kconfig are refactored out.

Any out-of-tree driver using the removed interface can be easily adapted
by copying the following implementations into the driver as private
functions:

 - `hci_driver.h:BT_HCI_EVT_FLAG_RECV_PRIO`
 - `hci_driver.h:BT_HCI_EVT_FLAG_RECV`
 - `hci_driver.h:bt_hci_evt_get_flags`
 - `hci_raw.c:bt_recv_prio`

In combination these symbols function as a interface adapter. These
symbols will be removed in this PR in subsequent commits.

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