dt-bindings: display: bridge: Remove deprecated dw_hdmi.txt

dw_hdmi.txt has been replaced with synopsys,dw-hdmi.yaml, and all
references to the old file have been converted. Remove it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Rob Herring <robh@kernel.org>