staging: rtl8188eu: remove unused rtw_calculate_wlan_pkt_size_by_attribue()

The function rtw_calculate_wlan_pkt_size_by_attribue() also defined as
rtw_wlan_pkt_size() is never used, so remove it. Discovered by cppcheck.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>