Main: remove unused global variable "main_thread"

src/Main.cxx

@@ -109,8 +109,6 @@ static constexpr unsigned DEFAULT_BUFFER_BEFORE_PLAY = 10;
 
 static constexpr Domain main_domain("main");
 
-ThreadId main_thread;
-
 Instance *instance;
 
 static StateFile *state_file;
@@ -412,8 +410,6 @@ int mpd_main(int argc, char *argv[])
 		return EXIT_FAILURE;
 	}
 
-	main_thread = ThreadId::GetCurrent();
-
 	instance = new Instance();
 	instance->event_loop = new EventLoop();
 

src/Main.hxx

@@ -20,12 +20,9 @@
 #ifndef MPD_MAIN_HXX
 #define MPD_MAIN_HXX
 
-class ThreadId;
 class EventLoop;
 struct Instance;
 
-extern ThreadId main_thread;
-
 extern Instance *instance;
 
 /**