UI: Removed unused static AddProjectorMenuMonitors declaration

e832b42 made AddProjectorMenuMonitors a member function of OBSBasic, but
the (now unused) declaration of the static function never got removed.

UI/window-basic-main.cpp

@@ -1861,9 +1861,6 @@ void OBSBasic::ResetOutputs()
 	}
 }
 
-static void AddProjectorMenuMonitors(QMenu *parent, QObject *target,
-				     const char *slot);
-
 #define STARTUP_SEPARATOR \
 	"==== Startup complete ==============================================="
 #define SHUTDOWN_SEPARATOR \