drm: rcar-du: Drop leftovers variables from Makefile

Commit 841281fe ("drm: rcar-du: Drop LVDS device tree backward
compatibility") has removed device tree overlay sources used for
backward compatibility with old bindings, but forgot to remove related
variables from the Makefile. Fix it.

Fixes: 841281fe ("drm: rcar-du: Drop LVDS device tree backward compatibility")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>