cfg80211: remove ieee80211_get_he_sta_cap()

This function turned out to be too easy to misuse since it
doesn't consider the interface type. Remove it now that we
no longer use it in mac80211.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210618133832.8c9c72f914b0.I68e9c0626dc77a0f67f238a05ae16a0b77b09895@changeid


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