bwn: remove unused ic_headroom

Unlike bwi(4), bwn(4) does not rely on ic_headroom (despite having it
set) but splits the bwn_txhdr (first) segment into its own transaction.
Remove ic_headroom to avoid net80211 troubles with not enough space in
the mbuf around ieee80211_mbuf_adjust().

PR: 275616
MFC after: 3 days