fix(video): remove invalid isDmaBuf override on GstDmaBufVideoBuffer

salvage: rebased of Ryanf55#14365 (credit @Ryanf55) after HwBuffers layout moved under dmabuf/.
Base GstHwVideoBuffer / QHwVideoBuffer no longer declare isDmaBuf(), so override fails to compile.

Author: Bartok9

src/VideoManager/VideoReceiver/GStreamer/HwBuffers/dmabuf/GstDmaBufVideoBuffer.h

@@ -19,8 +19,6 @@ class GstDmaBufVideoBuffer final : public GstHwVideoBuffer
     GstDmaBufVideoBuffer(GstSample* sample, const GstVideoInfo& videoInfo, const QVideoFrameFormat& format,
                          EGLDisplay eglDisplay);
 
-    bool isDmaBuf() const override { return true; }
-
     QVideoFrameTexturesUPtr mapTextures(QRhi& rhi, QVideoFrameTexturesUPtr& oldTextures) override;
     bool validatePlaneHandles() const override;