revert https://github.com/MusicPlayerDaemon/MPD/commit/c5e607a3107c3ec2592f5785db4c9b55faefcaae

to revisit when base libcxx is updated ?
Index: src/event/FineTimerEvent.hxx
--- src/event/FineTimerEvent.hxx.orig
+++ src/event/FineTimerEvent.hxx
@@ -7,7 +7,7 @@
 #include "Chrono.hxx"
 #include "event/Features.h"
 #include "util/BindMethod.hxx"
-#include "util/IntrusiveTreeSet.hxx"
+#include <boost/intrusive/set_hook.hpp>
 
 #include <cassert>
 
@@ -25,7 +25,7 @@ class EventLoop;
  * as thread-safe.
  */
 class FineTimerEvent final :
-	public IntrusiveTreeSetHook<IntrusiveHookMode::AUTO_UNLINK>
+	public boost::intrusive::set_base_hook<boost::intrusive::link_mode<boost::intrusive::auto_unlink>>
 {
 	friend class TimerList;
 
