pipeline: ipu3: Drop unneeded forward declarations

Multiple classes are forward-declared in frames.h, without being used in
that header file. Drop the unneeded forward declarations.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>

src/libcamera/pipeline/ipu3/frames.h

@@ -19,11 +19,7 @@
 namespace libcamera {
 
 class FrameBuffer;
-class IPAProxy;
-class PipelineHandler;
 class Request;
-class V4L2VideoDevice;
-struct IPABuffer;
 
 class IPU3Frames
 {