Bluetooth: Remove bt_read_static_addr() "hack"

This function was used to shortcut HCI for combined host + controller
builds. It doesn't provide much value and adds complexity to the HCI
driver interface, so just remove it. This means vendor-specific HCI
commands is now the only way for the host to access the same
information.

Signed-off-by: Johan Hedberg <johan.hedberg@gmail.com>