staging: rtl8723bs: remove unused variable declaration in include/rtw_security.h

staging: rtl8723bs: remove unused variable declaration in include/rtw_security.h

fix following kernel test robot warning:

   At top level:
   drivers/staging/rtl8723bs/include/rtw_security.h:336:28:
warning: 'K' defined but not used [-Wunused-const-variable=]
     336 | static const unsigned long K[64] = {
         |                            ^

Signed-off-by: Fabio Aiuto <fabioaiuto83@gmail.com>
Link: https://lore.kernel.org/r/819680ab87bbe543aba966f1fe47b209d3d8a69f.1617962215.git.fabioaiuto83@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>