libcamera: Drop unneeded empty default constructors and destructors

A few default constructors and destructors are needlessly defined as
empty. Drop them.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Umang Jain <email@uajain.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/signal.h

@@ -38,7 +38,6 @@ template<typename... Args>
 class Signal : public SignalBase
 {
 public:
-	Signal() {}
 	~Signal()
 	{
 		disconnect();