omxvideodec: Remove redundant code
omxvideodec: Remove redundant code

The pool reference was never used, besides it may leads to warning if unref a
NULL pool got from downstream.

refs #102576
Signed-off-by: Anh Ha <3c37b7c0454e9510fb1f952fa96c522d6e8c2a89@rvc.renesas.com>

renesas-rcar/gst-omx,renesas-rcar/gst-omx,renesas-rcar/gst-omx