mac80211: remove unused and unneeded remove_sta_debugfs callback

mac80211: remove unused and unneeded remove_sta_debugfs callback

The remove_sta_debugfs callback in struct rate_control_ops is no longer
used by any driver, as there is no need for it (the debugfs directory is
already removed recursivly by the mac80211 core.)  Because no one needs
it, just remove it to keep anyone else from accidentally using it in the
future.

Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20190612142658.12792-5-gregkh@linuxfoundation.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>