drm/docs: Remove the rcar alpha from the csv file

Now that we moved the rcar-du DRM driver has been switched to the generic
alpha property, remove the former property documentation from the
deprecated CSV file.
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/04be5e2256aa8d33d9521a68a10f0b73a24f8040.1523432341.git-series.maxime.ripard@bootlin.com

Documentation/gpu/kms-properties.csv

@@ -98,5 +98,4 @@ radeon,DVI-I,“coherent”,RANGE,"Min=0, Max=1",Connector,TBD
 ,,"""underscan vborder""",RANGE,"Min=0, Max=128",Connector,TBD
 ,Audio,“audio”,ENUM,"{ ""off"", ""on"", ""auto"" }",Connector,TBD
 ,FMT Dithering,“dither”,ENUM,"{ ""off"", ""on"" }",Connector,TBD
-rcar-du,Generic,"""alpha""",RANGE,"Min=0, Max=255",Plane,TBD
 ,,"""colorkey""",RANGE,"Min=0, Max=0x01ffffff",Plane,TBD