ARM: dts: imx6dl-aristainetos2_4: drop redundant 'power-on-delay' property

LG4573 panel bindings do not allow 'power-on-delay' property.  Linux
driver does not use it, either.  Reported by dtbs_check:

  imx6dl-aristainetos2_4.dtb: display@0: Unevaluated properties are not allowed ('power-on-delay' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>