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.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8723bs/os_dep/ioctl_cfg80211.c

@@ -154,17 +154,6 @@ static struct ieee80211_supported_band *rtw_spt_band_alloc(
 
 static void rtw_spt_band_free(struct ieee80211_supported_band *spt_band)
 {
-	u32 size = 0;
-
-	if (!spt_band)
-		return;
-
-	if (spt_band->band == NL80211_BAND_2GHZ)
-	{
-		size = sizeof(struct ieee80211_supported_band)
-			+ sizeof(struct ieee80211_channel)*RTW_2G_CHANNELS_NUM
-			+ sizeof(struct ieee80211_rate)*RTW_G_RATES_NUM;
-	}
 	kfree(spt_band);
 }