ath9k: remove unused min rate calculation code

This is not used, and when we need to get the lowest rate
we should simply use mac80211's own rate_lowest_index(sband, sta).

Cc: Derek Smithies <derek@indranet.co.nz>
Cc: Chittajit Mitra <Chittajit.Mitra@Atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>