[PATCH] libertas: remove adapter->txrate

The value of txrate was only set by a CMD_802_11_TX_RATE_QUERY command,
but there was no code in the driver that ever issued this command.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>