staging: rtl8821ae: deal with unused functions in halbtcoutsrc.c

This commit fixes the compiler -Wunused-function warning. Several
functions had to be removed.

Removed symbols

    halbtc_is_hw_mailbox_exist
    halbtc_set_macreg
    halbtc_get_macreg
    halbtc_under_ips

Signed-off-by: Konrad Zapalowicz <bergo.torino@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>