Revert r330471 (MFC of r311861), since it results in compile errors
like:

sys/net80211/ieee80211_tdma.c:179: error: 'IEEE80211_MODE_VHT_2GHZ' undeclared (first use in this function)
sys/net80211/ieee80211_tdma.c:180: error: 'IEEE80211_MODE_VHT_5GHZ' undeclared (first use in this function)

The IEEE80211_MODE_VHT_2GHZ and IEEE80211_MODE_VHT_5GHZ enum values are
defined in r310147, but that commit cannot be MFCd as-is, because it
likely breaks ABI.