staging: rtl8723bs: os_dep: remove set but not used 'size'

staging: rtl8723bs: os_dep: remove set but not used 'size'

And also remove the NULL check before kfree() because kfree() can handle
NULL pointers correctly.

Fix the following gcc warning:

drivers/staging/rtl8723bs/os_dep/ioctl_cfg80211.c:157:6: warning:
variable ‘size’ set but not used [-Wunused-but-set-variable]
  u32 size = 0;
      ^~~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200427032342.27211-3-yanaijie@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>