dt-bindings: display: remove backlight node from panel examples

The examples for these panel drivers have a backlight node in addition to
the actual panel node. However the exact backlight is outside the scope of
this binding and should be dropped from the example.

Link: https://lore.kernel.org/linux-devicetree/[e-mail]/
Suggested-by: Rob Herring <[e-mail]>
Signed-off-by: Luca Ceresoli <[e-mail]>
Acked-by: David Lechner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/display/ilitek,ili9486.yaml

···
   - |
     #include <dt-bindings/gpio/gpio.h>
 
-    backlight: backlight {
-            compatible = "gpio-backlight";
-            gpios = <&gpio 22 GPIO_ACTIVE_HIGH>;
-    };
     spi {
             #address-cells = <1>;
             #size-cells = <0>;

Documentation/devicetree/bindings/display/panel/ilitek,ili9163.yaml

···
   - |
     #include <dt-bindings/gpio/gpio.h>
 
-    backlight: backlight {
-            compatible = "gpio-backlight";
-            gpios = <&gpio 22 GPIO_ACTIVE_HIGH>;
-    };
     spi {
             #address-cells = <1>;
             #size-cells = <0>;

Documentation/devicetree/bindings/display/sitronix,st7735r.yaml

···
   - |
     #include <dt-bindings/gpio/gpio.h>
 
-    backlight: backlight {
-            compatible = "gpio-backlight";
-            gpios = <&gpio 44 GPIO_ACTIVE_HIGH>;
-    };
-
     spi {
             #address-cells = <1>;
             #size-cells = <0>;