usb: device: remove USB H4 Bluetooth function and sample

The Bluetooth HCI USB transport layer implementation is provided by
"subsys/usb/device/class/bluetooth.c". The USB H4 Bluetooth function
implements a non-standard transport layer. There is no known host-side
equivalent that uses this protocol.

Note that the H4 protocol functionality is also provided by the
"subsys/usb/device/class/bluetooth.c".

Since that there are no real USB H4 Bluetooth users, remove the
implementation and sample without deprecation.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>