staging: rtl8723bs: core: remove set but not used 'listen_interval'

staging: rtl8723bs: core: remove set but not used 'listen_interval'

Fix the following gcc warning:

drivers/staging/rtl8723bs/core/rtw_mlme_ext.c:1173:18: warning: variable
‘listen_interval’ set but not used [-Wunused-but-set-variable]
  u16 capab_info, listen_interval;
                  ^~~~~~~~~~~~~~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200427032342.27211-6-yanaijie@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>