wireless: remove obsolete chan no/center freq conversion functions

There are a number of functions that shouldn't really
be used when modern functions that take the band are
available in cfg80211. Remove these, but for now keep
 * ieee80211_freq_to_dsss_chan and
 * ieee80211_dsss_chan_to_freq
as they're used in older drivers.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>