qtnfmac: drop nonexistent function declaration

Function qtnf_classify_skb_no_mbss has been used for debug
during early stage of development. Drop its declaration.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>