wifi: ieee80211: remove ieee80211_next_tbtt_present()

This is actually completely equivalent to the other function
ieee80211_is_s1g_short_beacon(), but open-codes the logic.
Implement the necessary logic in ieee80211_is_s1g_short_beacon()
and remove ieee80211_next_tbtt_present().

Link: https://msgid.link/20240515093852.774ced74dea8.I152525b4cff6e6a25be6c48fe6a4b89f17bab8a9@changeid


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