Staging: rtl8723au: Remove function rtw_enqueue_{recvbuf23a/recvbuf23a_to_head}

The functions rtw_enqueue_recvbuf23a and rtw_enqueue_recvbuf23a_to_head
are never used anywhere in the kernel. So, remove their definition and
prototype.
Grepped to find occurences.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>