event_pipe: replaced PIPE_EVENT_SIGNAL with main_notify

There is only one location using PIPE_EVENT_SIGNAL: to synchronize
player_command() with player_command_finished().  Use the "notify"
library instead of the event_pipe here.

src/event_pipe.h

@@ -24,10 +24,6 @@
 #include <glib.h>
 
 enum pipe_event {
-	/** the default event: the main thread is waiting for somebody,
-	    and this event wakes up the main thread */
-	PIPE_EVENT_SIGNAL = 0,
-
 	/** database update was finished */
 	PIPE_EVENT_UPDATE,
 
@@ -66,8 +62,6 @@ void event_pipe_emit(enum pipe_event event);
  */
 void event_pipe_emit_fast(enum pipe_event event);
 
-void event_pipe_signal(void);
-
 void event_pipe_wait(void);
 
 #endif /* MAIN_NOTIFY_H */