rtlwifi: remove unneeded function _rtl_dump_channel_map()

Now _rtl_dump_channel_map() does not do any actual
thing using the channel. So remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>