rtw88: Delete useless kfree code

The parameter of kfree function is NULL, so kfree code is useless, delete it.

Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201216130442.13869-1-zhengyongjun3@huawei.com