dt-bindings: sound: Remove unused 'linux,hdmi-audio'

The binding was added in 2013 and has had no driver since 2015.

Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: alsa-devel@alsa-project.org
Link: https://lore.kernel.org/r/20200826184851.3431531-1-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/sound/hdmi.txt

-Device-Tree bindings for dummy HDMI codec
-
-Required properties:
-	- compatible: should be "linux,hdmi-audio".
-
-CODEC output pins:
-  * TX
-
-CODEC input pins:
-  * RX
-
-Example node:
-
-	hdmi_audio: hdmi_audio@0 {
-		compatible = "linux,hdmi-audio";
-	};