iwlwifi: remove 60 Mbps from sband bitrates table

ieee80211_supported_band is supposed to only contain legacy rates in the
bitrates table (HT rates go in the ieee80211_sta_ht_cap substruct).  Make
iwlwifi driver obey this restriction by removing the 60 Mbps rate. Also, clean
up a few pieces of other code that formerly relied on 60 Mbps being in
sband->bitrates.

Signed-off-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>