libcamera: framebuffer: Make FrameBuffer::cancel() private

FrameBuffer::cancel() is not meant to be used by applications. Move it
to the FrameBuffer::Private class.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Dorota Czaplejewicz <dorota.czaplejewicz@puri.sm>
[Kieran: Handle rebase]
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/framebuffer.h

@@ -71,8 +71,6 @@ public:
 
 	std::unique_ptr<Fence> releaseFence();
 
-	void cancel() { metadata_.status = FrameMetadata::FrameCancelled; }
-
 private:
 	LIBCAMERA_DISABLE_COPY_AND_MOVE(FrameBuffer)