rtlwifi: remove useless code

Remove useless local variables last_read_point and last_txw_point and
the code related.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: David S. Miller <davem@davemloft.net>