ARM: dts: imx8mq-evk: Remove u-boot,off-on-delay-us property

Commit 247bbeb7

 ("ARM: dts: imx8m: increase off-on delay on the SD Vcc
regulator") caused the imx8mq-evk board to not be able to store the
environment variables in the SD card.

Remove the u-boot,off-on-delay-us property to fix the regression.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>