staging/rtl8192u: remove unused function

Remove unused function N_DBPSOfRate. This function was only used by
function ComputeTxTime that was removed in the previous
commit 742728f9 ("staging: rtl8192u: remove unused function.")

Signed-off-by: Nicolas Joseph <nicolas.joseph@homecomputing.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>