staging: rtl8723bs: remove unused variable in core/rtw_cmd.c

staging: rtl8723bs: remove unused variable in core/rtw_cmd.c

fix following kernel test robot warning:

   drivers/staging/rtl8723bs/core/rtw_cmd.c:
	In function 'rtw_cmd_thread':
>> drivers/staging/rtl8723bs/core/rtw_cmd.c:390:16:
warning: variable 'cmd_start_time' set
	but not used [-Wunused-but-set-variable]
     390 |  unsigned long cmd_start_time;
         |                ^~~~~~~~~~~~~~

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Fabio Aiuto <fabioaiuto83@gmail.com>
Link: https://lore.kernel.org/r/509eeb1c4a7fcfc37b69cb8578ea146b95b50864.1617962215.git.fabioaiuto83@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>