dt-bindings: display: drop unused simple-panel.txt

There are no more references to simple-panel.txt.
Delete it.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408195109.32692-29-sam@ravnborg.org

Documentation/devicetree/bindings/display/panel/simple-panel.txt

-See panel-common.yaml in this directory.