wifi: mac80211: Clean up debugfs_key deadcode

wifi: mac80211: Clean up debugfs_key deadcode

The last use of ieee80211_debugfs_key_sta_del() was removed in 2007 by
commit 11a843b7e160 ("[MAC80211]: rework key handling")

The last use of ieee80211_debugfs_key_add_mgmt_default() was removed
in 2010 by
commit f7e0104c1a4e ("mac80211: support separate default keys")

The last use of ieee80211_debugfs_key_add_beacon_default() was
removed in 2020 by
commit e5473e80d467 ("mac80211: Support BIGTK configuration for Beacon
protection")

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://patch.msgid.link/20241224013257.185742-2-linux@treblig.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>