mac80211: remove kdoc references to IEEE80211_HW_HOST_GEN_BEACON_TEMPLATE

IEEE80211_HW_HOST_GEN_BEACON_TEMPLATE was made unnecessary in
the recent revamp on beacon configuration.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>