rt2x00: Remove deprecated ieee80211_rx_status->qual usage

ieee80211_rx_status->qual has been marked deprecated.
This allows us to remove several functions and fields which
were used to calculate a reasonable value for it.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>