Remove btm_ble_advertiser_hci_interface

The only place it was used is guarded by a flag
in unused code.  Remove that usage and the rest
of the code.

bool legacy_advertising_in_use = false; <--- This flag is never set.
void btm_ble_advertiser_notify_terminated_legacy(uint8_t status,
                                                 uint16_t connection_handle) {
  if (BleAdvertiserHciInterface::Get() && legacy_advertising_in_use) {
    ((BleAdvertiserLegacyHciInterfaceImpl*)BleAdvertiserHciInterface::Get())
        ->OnAdvertisingSetTerminated(status, connection_handle);
  }
}

Bug: 301661850
Test: mma -j32
Flag: EXEMPT, dead code removal
Change-Id: I17a936abb5cd1b64b0cd87d6c6266efa3f36f6e7