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

There are no more references to simple-panel.txt.
Delete it.

Signed-off-by: Sam Ravnborg <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Cc: Thierry Reding <[e-mail]>
Cc: Sam Ravnborg <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

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

···
-See panel-common.yaml in this directory.