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.org


Signed-off-by: Rob Herring <robh@kernel.org>