Bluetooth: Remove hci_recv_stream_fragment function

The hci_recv_stream_fragment function should have never been introduced
in the first place. The Bluetooth core does not need to know anything
about the HCI transport protocol.

With all transport protocol specific detailed moved back into the
drivers where they belong (mainly generic USB and UART drivers), this
function can now be removed.

This reduces the size of hci_dev structure and also removes an exported
symbol from the Bluetooth core module.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>